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Input offset voltage $V_{OS}$ Your problem might be input offset voltage or $V_{OS}$. This can be several milli-volts and manifests as this modification to your original circuit (in red): - So, fo...
Answer
#4: Post edited
by
Andy aka
·
2020-06-27T20:42:16Z (almost 4 years ago)
# Input offset voltageYour first problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit: -- 
So, for an op-amp like the LM358 or LM324, you can expect a typical \$V_{OS}\$ of about 2 mV. This can add-to or subtract from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.Of course, for AC signal amplification this isn't really a problem but if you need DC accuracy you need to choose an appropriate op-amp. Your error limit might be 1% and therefore 1% of 10 mV is 100 μV so, if you need this accuracy you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.But also be aware that the specified \$V_{OS}\$ will drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -- 
Highlighted in red is the typical offset voltage of ±20 μV but, at 25 °C to might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset might drift with temperature.So if your op-amp is subject to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -$$ 65\text{ μV} + 45\text{ μV} = 110\text{ μV}$$This, might be a little too-much and so you should choose a better op-amp.# Input bias currentSo let's say you did choose a decent op-amp and expected to see an output voltage of 1.01 volts ± 10 mV but this still didn't happen. The next cause to consider is input bias currents.Input bias currents flow from/into the op-amp inputs and can be in the 10 to 100 nano amp range but, if you are using quite high value resistors, then that bias current multiplied by the effective parallel combination of \$R_F\$ and \$R_G\$ can also produce a significant offset. That offset can be regarded as being in exactly the same position as \$V_{OS}\$.- Using the LM324 as an example: -
- 
As you should be able to see (red box), the worst case input bias current is ±100 nA and, if this flows through a 10 kΩ resistor, the volt-drop would be ±0.001 volts. As before, when multiplied by the gain of 101, the output error becomes ±0.101 volts.- However, if you added a 10 kΩ resistor in series with the input signal as shown below, you do get an improvement: -
- 
Look at the green box in the LM324 specification - it has an input offset current (\$I_{OS}\$) of 30 nA. This is the maximum difference between the bias currents of the two op-amp inputs and is referred to as an offset because of this.- With the addition of the extra 10 kΩ resistor, the offset voltage created by the input bias currents are nearly cancelled but, they are not minimized to zero. In effect, we can now regard an input offset current of 30 nA flowing through the added 10 kΩ resistor and this, produces an offset voltage of 0.3 mV. Multiply this by the gain of 101 to get an output error of ±0.0303 volts.
Clearly, if we were looking for an error no greater than 1% of 1.01 volts on the output, we still haven't achieved this using an op-amp with similar bias currents as the LM324 so a better op-amp is needed- -----
- # All together now
Input bias currents (\$I_B\$ & \$I_{OS}\$) together with input offset voltages (\$V_{OS}\$) do not work separately - they both occur together so, you have to take care when choosing the op-amp when needing to accurately amplify small DC signals.
- # Input offset voltage \$V_{OS}\$
- Your problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit (in red): -
- 
- So, for an op-amp like the LM358 or LM324, you can expect a typical \$V_{OS}\$ of about 2 mV. This can add-to or subtract-from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.
- For AC signal amplification this isn't really a problem but, if you need DC accuracy, then you should choose a better op-amp. Your error limit might be 1% and, 1% of 10 mV is 100 μV so, if you need this accuracy, you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.
- But, also be aware that \$V_{OS}\$ can drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -
- 
- Highlighted in red is the typical \$V_{OS}\$ of ±20 μV but, at 25 °C it might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset could drift with temperature.
- So if your op-amp "sees" to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -
- $$\boxed{65\text{ μV} + 45\text{ μV} = 110\text{ μV}}$$
- If this is too-much you should choose a better op-amp.
- # Input currents \$I_B\$ and \$I_{OS}\$
- So let's say you did choose a decent op-amp and expected to see an output voltage within 1.01 volts ± 10 mV but, this still didn't happen. The next problem to consider is input bias current (\$I_B\$).
- \$I_B\$ flows from/into the op-amp inputs and can be commonly in the 1 to 100 nano amp range but, if you are using quite high-value op-amp resistors (\$R_F\$ and \$R_G\$), then that bias current multiplied by the effective parallel combination of \$R_F\$ and \$R_G\$ can "generate" a significant offset voltage.
- $$\boxed{\text{That offset can be regarded as being in exactly the same position as }V_{OS}}$$
- Using the LM324 as an example: -
- 
- As you should be able to see (red box), the worst case input bias current is ±100 nA and, if this flows through a 10 kΩ resistor, the volt-drop would be ±0.001 volts. When multiplied by the circuit gain of 101, the output error becomes ±0.101 volts.
- However, if you added a 10 kΩ resistor in series with the input signal as shown below, you do get an improvement: -
- 
- Look at the green box in the LM324 specification - it has an input offset current (\$I_{OS}\$) of 30 nA. This is the maximum difference between the bias currents of the two op-amp inputs and is called an offset because of this.
- With the addition of the extra 10 kΩ resistor, the offset voltage created by the input bias currents are nearly cancelled but, they are not minimized to zero. In effect, we can now regard an input offset current of 30 nA flowing through the added 10 kΩ resistor and this, produces an offset voltage of 0.3 mV. Multiply this by the gain of 101 to get an output error of ±0.0303 volts.
- Clearly, if we were looking for an error no greater than 1% of 1.01 volts on the output, we still haven't achieved this using an op-amp with similar bias currents as the LM324 so a better op-amp is needed.
- -----
- # All together now
- Input bias currents (\$I_B\$ & \$I_{OS}\$) and input offset voltage (\$V_{OS}\$) do not work separately - they both occur together and are indistinguishable so, if need to accurately amplify small DC signals, you have to choose the appropriate op-amp.
#3: Post edited
by
Andy aka
·
2020-06-27T14:21:22Z (almost 4 years ago)
- # Input offset voltage
- Your first problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit: -
- 
So, for an op-amp like the LM358 or LM324, it has a typical \$V_{OS}\$ of about 2 mV. This can therefore add-to or subtract from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.- Of course, for AC signal amplification this isn't really a problem but if you need DC accuracy you need to choose an appropriate op-amp. Your error limit might be 1% and therefore 1% of 10 mV is 100 μV so, if you need this accuracy you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.
But be also aware that the specified \$V_{OS}\$ will drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -- 
- Highlighted in red is the typical offset voltage of ±20 μV but, at 25 °C to might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset might drift with temperature.
- So if your op-amp is subject to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -
- $$ 65\text{ μV} + 45\text{ μV} = 110\text{ μV}$$
This, with might be a little too-much and so you might choose a better op-amp.- # Input bias current
- So let's say you did choose a decent op-amp and expected to see an output voltage of 1.01 volts ± 10 mV but this still didn't happen. The next cause to consider is input bias currents.
Input bias currents flow from/into the op-amp inputs and can be in the 10 to 100 nano amp range but, if you are using quite high value resistors such as 1 MΩ or more, then that bias current multiplied by the effective parallel combination of \$R_F\$ and \$R_G\$ can also produce a significant offset. That offset can be regarded as being in exactly the same position as \$V_{OS}\$.- Using the LM324 as an example: -
- 
As you should be able to see (red box), the worst case input bias current is ±100 nA and, if this flowed through a measly 10 kΩ resistor, the volt-drop would be ±0.001 volts. As before, when multiplied by the gain of 101, the output error becomes ±0.101 volts.- However, if you added a 10 kΩ resistor in series with the input signal as shown below, you do get an improvement: -
- 
- Look at the green box in the LM324 specification - it has an input offset current (\$I_{OS}\$) of 30 nA. This is the maximum difference between the bias currents of the two op-amp inputs and is referred to as an offset because of this.
With the addition of the extra 10 kΩ resistor, the offset voltage created by the input bias currents are minimized but, they are not minimized to zero. In effect, we can now regard just an offset of 30 nA flowing through to extra 10 kΩ resistor and this produces an offset voltage of 0.3 mV. Multiply this by the gain of 101 to get an output error of ±0.0303 volts.- Clearly, if we were looking for an error no greater than 1% of 1.01 volts on the output, we still haven't achieved this using an op-amp with similar bias currents as the LM324 so a better op-amp is needed
- -----
- # All together now
Input bias currents (\$I_B\$ & \$I_{OS}\$) together with input offset voltages (\$V_{OS}\$) do not work separately - they can both occur together so, you have to take care when choosing the op-amp when needing to accurately amplify small DC signals.
- # Input offset voltage
- Your first problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit: -
- 
- So, for an op-amp like the LM358 or LM324, you can expect a typical \$V_{OS}\$ of about 2 mV. This can add-to or subtract from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.
- Of course, for AC signal amplification this isn't really a problem but if you need DC accuracy you need to choose an appropriate op-amp. Your error limit might be 1% and therefore 1% of 10 mV is 100 μV so, if you need this accuracy you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.
- But also be aware that the specified \$V_{OS}\$ will drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -
- 
- Highlighted in red is the typical offset voltage of ±20 μV but, at 25 °C to might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset might drift with temperature.
- So if your op-amp is subject to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -
- $$ 65\text{ μV} + 45\text{ μV} = 110\text{ μV}$$
- This, might be a little too-much and so you should choose a better op-amp.
- # Input bias current
- So let's say you did choose a decent op-amp and expected to see an output voltage of 1.01 volts ± 10 mV but this still didn't happen. The next cause to consider is input bias currents.
- Input bias currents flow from/into the op-amp inputs and can be in the 10 to 100 nano amp range but, if you are using quite high value resistors, then that bias current multiplied by the effective parallel combination of \$R_F\$ and \$R_G\$ can also produce a significant offset. That offset can be regarded as being in exactly the same position as \$V_{OS}\$.
- Using the LM324 as an example: -
- 
- As you should be able to see (red box), the worst case input bias current is ±100 nA and, if this flows through a 10 kΩ resistor, the volt-drop would be ±0.001 volts. As before, when multiplied by the gain of 101, the output error becomes ±0.101 volts.
- However, if you added a 10 kΩ resistor in series with the input signal as shown below, you do get an improvement: -
- 
- Look at the green box in the LM324 specification - it has an input offset current (\$I_{OS}\$) of 30 nA. This is the maximum difference between the bias currents of the two op-amp inputs and is referred to as an offset because of this.
- With the addition of the extra 10 kΩ resistor, the offset voltage created by the input bias currents are nearly cancelled but, they are not minimized to zero. In effect, we can now regard an input offset current of 30 nA flowing through the added 10 kΩ resistor and this, produces an offset voltage of 0.3 mV. Multiply this by the gain of 101 to get an output error of ±0.0303 volts.
- Clearly, if we were looking for an error no greater than 1% of 1.01 volts on the output, we still haven't achieved this using an op-amp with similar bias currents as the LM324 so a better op-amp is needed
- -----
- # All together now
- Input bias currents (\$I_B\$ & \$I_{OS}\$) together with input offset voltages (\$V_{OS}\$) do not work separately - they both occur together so, you have to take care when choosing the op-amp when needing to accurately amplify small DC signals.
#2: Post edited
by
Andy aka
·
2020-06-27T14:15:07Z (almost 4 years ago)
- Your first problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit: -
- 
- So, for an op-amp like the LM358 or LM324, it has a typical \$V_{OS}\$ of about 2 mV. This can therefore add-to or subtract from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.
- Of course, for AC signal amplification this isn't really a problem but if you need DC accuracy you need to choose an appropriate op-amp. Your error limit might be 1% and therefore 1% of 10 mV is 100 μV so, if you need this accuracy you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.
- But be also aware that the specified \$V_{OS}\$ will drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -
- 
- Highlighted in red is the typical offset voltage of ±20 μV but, at 25 °C to might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset might drift with temperature.
- So if your op-amp is subject to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -
- $$ 65\text{ μV} + 45\text{ μV} = 110\text{ μV}$$
This, with might be a little too-much and so you might choose a better op-amp.
- # Input offset voltage
- Your first problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit: -
- 
- So, for an op-amp like the LM358 or LM324, it has a typical \$V_{OS}\$ of about 2 mV. This can therefore add-to or subtract from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.
- Of course, for AC signal amplification this isn't really a problem but if you need DC accuracy you need to choose an appropriate op-amp. Your error limit might be 1% and therefore 1% of 10 mV is 100 μV so, if you need this accuracy you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.
- But be also aware that the specified \$V_{OS}\$ will drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -
- 
- Highlighted in red is the typical offset voltage of ±20 μV but, at 25 °C to might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset might drift with temperature.
- So if your op-amp is subject to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -
- $$ 65\text{ μV} + 45\text{ μV} = 110\text{ μV}$$
- This, with might be a little too-much and so you might choose a better op-amp.
- # Input bias current
- So let's say you did choose a decent op-amp and expected to see an output voltage of 1.01 volts ± 10 mV but this still didn't happen. The next cause to consider is input bias currents.
- Input bias currents flow from/into the op-amp inputs and can be in the 10 to 100 nano amp range but, if you are using quite high value resistors such as 1 MΩ or more, then that bias current multiplied by the effective parallel combination of \$R_F\$ and \$R_G\$ can also produce a significant offset. That offset can be regarded as being in exactly the same position as \$V_{OS}\$.
- Using the LM324 as an example: -
- 
- As you should be able to see (red box), the worst case input bias current is ±100 nA and, if this flowed through a measly 10 kΩ resistor, the volt-drop would be ±0.001 volts. As before, when multiplied by the gain of 101, the output error becomes ±0.101 volts.
- However, if you added a 10 kΩ resistor in series with the input signal as shown below, you do get an improvement: -
- 
- Look at the green box in the LM324 specification - it has an input offset current (\$I_{OS}\$) of 30 nA. This is the maximum difference between the bias currents of the two op-amp inputs and is referred to as an offset because of this.
- With the addition of the extra 10 kΩ resistor, the offset voltage created by the input bias currents are minimized but, they are not minimized to zero. In effect, we can now regard just an offset of 30 nA flowing through to extra 10 kΩ resistor and this produces an offset voltage of 0.3 mV. Multiply this by the gain of 101 to get an output error of ±0.0303 volts.
- Clearly, if we were looking for an error no greater than 1% of 1.01 volts on the output, we still haven't achieved this using an op-amp with similar bias currents as the LM324 so a better op-amp is needed
- -----
- # All together now
- Input bias currents (\$I_B\$ & \$I_{OS}\$) together with input offset voltages (\$V_{OS}\$) do not work separately - they can both occur together so, you have to take care when choosing the op-amp when needing to accurately amplify small DC signals.
#1: Initial revision
by
Andy aka
·
2020-06-27T13:39:44Z (almost 4 years ago)
Your first problem might be input offset voltage or \$V_{OS}\$. This can be several milli-volts and manifests as this modification to your original circuit: -
So, for an op-amp like the LM358 or LM324, it has a typical \$V_{OS}\$ of about 2 mV. This can therefore add-to or subtract from your 10 mV input signal. If it is subtractive then your *effective* input is now 8 mV and, with a gain of 101 you get an output voltage of 808 mV.
Of course, for AC signal amplification this isn't really a problem but if you need DC accuracy you need to choose an appropriate op-amp. Your error limit might be 1% and therefore 1% of 10 mV is 100 μV so, if you need this accuracy you should look for an op-amp with a \$V_{OS}\$ better than ±100 μV.
But be also aware that the specified \$V_{OS}\$ will drift with temperature. Take a device like the AD8605 (I'm choosing that because I know them like the back of my hand): -
Highlighted in red is the typical offset voltage of ±20 μV but, at 25 °C to might be as high as ±65 μV. OK, ±65 μV might be OK for the error budget but, take a look at the number highlighted in green. This tells you how much that offset might drift with temperature.
So if your op-amp is subject to a 10 °C change in temperature, you can add-on another 45 μV to your expected \$V_{OS}\$ and now the figure becomes: -
$$ 65\text{ μV} + 45\text{ μV} = 110\text{ μV}$$
This, with might be a little too-much and so you might choose a better op-amp.