Part Number: THS4140
Adding a programmable BIT voltage (like from a DAC) to a summing amplifier stage (In front of an ADC) is straightforward. It's just a second summing node from a voltage source through an impedance. The goal being a short BIT sequence to add, say, a 0.5V test voltage on top of a signal already there just to see if the gain from a sensor stage is operating properly.
However, when using a fully-differential stage like a THS4140, there is no straightforward way of doing this. There doesn't seem to be a clean way of doing this short of using analog switches to switch in/out a separate voltage source instead of the differential sensor. (There isn't a fully-differential "summing" opamp)
Has anyone seen anything clever that I might be missing? I hate to convert to a non-differential opamp just to be able to add BIT capability.
The same concept can be applied to an FDA as you can see below.
In regular configuration:
With an added offset:
Please let me know if you have any questions.
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In reply to Hasan Babiker31:
Thanks for the response.
Yes, this seems to work with a unity gain, but unless I have a problem with my simulation, the offset seems to shift my attenuation by a small percentage.
We were attenuating by about 5%, and with a 0.25V addition on to a 1V input, the attenuation ends up wrong by about 0.5%. We might have something wrong in our sim though.
In reply to Jeff Mercure:
Can you send me your simulation files?
Our bad on the simulation. This method simulates fine now.
Thanks for your help
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