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Low voltage difference amplifier
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OM222O:
Hello
I need to be able to read a really small signal as a differential pair using an delta sigma ADC.
The adc itself allows for 2 differential channels but I want to eliminate the noise as much as possible using a filter before feeding it into the input of the amplifier (Something similar to this circuit which I found on a T.I datasheet):


my idea was to use a difference amplifier followed by a second order low pass filter to block any AC noise but this requires very low offset amplifiers such as the MAX4238. The signal itself is in the range of 10s of uV (measured across high current shunt using 2 wires of about 30cm length) so signal to noise ratio is pretty awful on the input side. I haven't been able to find a difference amplifier with low offset voltages so far and I know using external resistors is janky at best. I was considering having the footprint of the shunt resistor etched on the PCB so instead of using wires, I can directly touch it to the PCB to get a reading but that will be my last backup option. If anyone has a different approach to solving the issue or knows a difference amplifier with about 5uV max offset, please let me know, thanks.
mvs:

--- Quote from: OM222O on January 20, 2019, 10:14:33 am ---I know using external resistors is janky at best.

--- End quote ---
Yes, but you may look for precision resistor networks. They can have very good tolerance and tempco on resistance ratio.
iMo:
The ultra low offset/noise/tempco differential (output) opams are almost none existent. I've been struggling with it in a different thread on an AFE for 24++bit ADCs.

While reading posts on TI's engineering forum I've seen the TI's OPA388 has been recommended as the best front end opamp for the ADS1263 high-end. It is not a differential output opamp, however.
You may consider single ended ADC input, ie. with ANI- grounded, and you would need a small negative supply for the opamp as well.

Btw, there are a few ADCs which can work bipolar (ie AVcc=2.5V, AVss=-2.5V) and they have got a chopper PGA as well. Usually you do not need any AFE for it then, as your shunt is very low impedance.

Resistors: the LT5400-x series (quad resistors array on a chip) is low abs tempco (8ppm/C) with 0.2ppm/C matching tempco.
OM222O:

--- Quote from: imo on January 20, 2019, 12:57:04 pm ---The ultra low offset/noise/tempco differential (output) opams are almost none existent. I've been struggling with it in a different thread on an AFE for 24++bit ADCs.

While reading posts on TI's engineering forum I've seen the TI's OPA388 has been recommended as the best front end opamp for the ADS1263 high-end. It is not a differential output opamp, however.
You may consider single ended ADC input, ie. with ANI- grounded, and you would need a small negative supply for the opamp as well.

Btw, there are a few ADCs which can work bipolar (ie AVcc=2.5V, AVss=-2.5V) and they have got a chopper PGA as well. Usually you do not need any AFE for it then, as your shunt is very low impedance.

Resistors: the LT5400-x series (quad resistors array on a chip) is low abs tempco (8ppm/C) with 0.2ppm/C matching tempco.


--- End quote ---

The LT5400 seems a really solid option! I don't really need split supply ADC but thanks for the recommendation. the small negative supply already exists in the design but thanks for mentioning it. OPA2388 would be great for the task as well without being too expensive (2 of the MAX4238s will cost more but probably perform marginally better, I need to do more testing on that). You saved my day, thanks!
iMo:
LT5400 is not cheap, btw. :)
You may use the LT5400 for the R1-R4 with the OPAx388 (with some small -Vss) and the R5 and R6 are not critical, but they should be kept rather low res. The second opamp for the Vcm (Vref/2), the R divider at its input is not critical as the Vcm errors cancel out (diff ADC inputs).
PS: the Vcm opamp could be any standard RR low voltage one.
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