Tesatronic TTD20 DGND AGND unconnected
The TESATRONIC TTD20 is a precision length measuring instrument used in various industrial and laboratory settings. On the backside it provides a 15 pin interface for reading out signals:
I wanted to use this interface to connect to an external measurement PCB. Before designing it, I wanted to verify if all the functions are still working, since this is a rather old tool. First thing I noticed was that the 5V line on pin 12 did not supply any voltage.
From the schematic it looked like the references are tied together at the transformer. +-15V against AGND were correct, but against DGND I measured nothing. I measured between pin 9(AGND) and 12(DGND), it was open circuit. I thought something may be damaged inside, so I opened the device. I looked at the pin 12 on the connector and tried to track it, to see where it goes. Against a light source it can be seen this is a 2 layer board, so nothing is hidden on internal layers:
It seems that the DGND goes nowhere. Looking near the transformer, 3 linear converters are present(15,-15,5):
Here is the bottom side:
Measuring the ground pin, it turned out that all 3 of them were connected to AGND. The transformer itself had no markings that I could use to identify it, but it had 10 pins, and on the secondary side 3 out of the 5 pins were AGND as well. For this part my assumption would be that the 5V LDO may have failed as short.
The datasheet is not online and unfortunately I cannot upload files here. It was requested from Tesatronic, and they stopped selling this 20 years ago.
The surprising part to me is that there is no connection between AGND and DGND, and DGND seems to go nowhere. I see a lot of unpopulated parts, and was wondering could it be a specific configuration of the device where the digital supply is not present? Perhaps someone is familiar with this device?
1 answer
I'm not familiar with this device, but it is possible that different variants were made or were possible with the same PCB. Consider the documentation you have as general guidance, not necessarily exact. I've seen this a lot with old devices like that. (New devices solve this problem by not providing any documentation. Much better!)
The diagram does show the analog and digital grounds tied together, but it also shows the 5 V digital supply coming from a separate secondary, not just another tap from a single secondary. It looks like maybe the 5 V supply was originally intended to be isolated, but that in later units they decided to use a single common ground.
Go with what you've got. I'd try to power an add-on device externally anyway. In other words, don't use any of the supplies from this device. Use your own. I imagine you want to interface this device to a computer, so having your interface circuit get powered from the computer's USB seems like the obvious thing to do anyway.
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