# ADC differential filter for DC signals

I am trying to build an ADC module using the ADS1110 ADC and the TLS2272 op amp. This is more a problem of composition than a filter design problem.

The 16bit ADS1110 runs at the amazing speed of 15 SPS with a range of 0-4.096V. Unfortunately it is not a fully differential ADC and to reach 16bit on the whole range you need to use a Vref of 2.048V on the AIN- pin (see datasheet for more info). To restore the differential feature I am planning of using an opamp in differential mode, with a lp filter (fc=20Hz).

This is the circuit I came up with, but it's wrong, because the op amp is single supply and it can't detect if the signal ground noise ripple goes below 0V. Also note that the cutoff freq of this circuit is 200Hz not 20, ignore it.

I thought about some variations but I don't know which is the better one:

1) Remove the SK, use the freed opamp unit to buffer Vref, sum vref to both inputs of the differential filter

2) Remove the SK, use the freed opamp unit to sum the signal ground to Vref, feed this to the (-) input of the ADC

3) Something else

EDIT:

Main goal is to filter noise from both the signal and its ground, make the difference and feed it to the AIN+ of the ADC. Also, "signal" maybe is too much, I just need to read a fixed DC voltage.

Explanation of the Vref trick:

The ADC reads the (+) input within +/- 2V the (-) input. Since my signal ground is probably close to the ADC ground and the ADC can't read negative voltages I need to move the (-) input to 2V to use the full range with full precision.

• your LP cutoff is far too high. It needs to be lower than 15 Hz / 2 = 7.5 Hz. May 23, 2017 at 15:18
• So, what are the voltages you want to detect? You only mention that you need to get some negative ripple into the picture, but I'm not really seeing you well-define the specs of your overall system. Actual bandwidth (lower and upper frequencies) of the signal you want to observe would be very interesting, too! May 23, 2017 at 15:21
• Also, the ADS1110 has a differential input, and you don't explain why you don't use it as such. May 23, 2017 at 15:24
• Shoudn't it be 15 * 2? May 23, 2017 at 15:24
• It is written in the datasheet, you have only 15bit with differential input, and full 16 bit with the Vref trick. Here is a link of the explanation: ti.com/lit/an/sbaa133a/sbaa133a.pdf May 23, 2017 at 15:26

You over-engineered your module to the point that most people probably have real difficulty to understand, what you are trying to achieve. This how it can (and probably should be) simplified

1. This ADC is not really suitable for your application. You need something like AD7172-2 that can take handle (pseudo)-differential range of 5 V (with an external voltage reference). So search different manufacturers for suitable ADCa, ideally with integrated input buffers, but pay attention to input voltage limitations.
2. Delta Sigma ADCs have rather soft requirements for input filters. Passive filters are totally sufficient. That makes right opamp redundant.
3. Since you measure DC signal, your input filters are not as much about anti-aliasing but more about bandlimiting input noise , and do not need to be that tight. (see the link in 2. in case you still haven't)
4. Directly use differential inputs to measure your signal (assuming you followed recommendation 1), that's how they supposed to be used.Thus you get read of opamps that themselves can be the source of noise.

This is how your circuit should roughly look like

simulate this circuit – Schematic created using CircuitLab

Note that the values of the resistors and capacitors are placeholders and do not represent recommendation for your case. They have to be chosen in accordance with master sampling frequency (of the delta-sigma ADC) and input impedance of the ADC. If there are no input buffers, filter resistors must be low valued (no more than 20 Ohm)

• However, for anyone interested, I found some interesting readings that fits original problem: 1 2 May 24, 2017 at 15:23

Your front-end amplifier is a unity gain difference amplifier. You are free to choose the reference potential for differential to single-ended conversion. Tie R3 to your 2.5 Reference not AGND.

Your CMRR will depend on the matching of your 5k resistors. See http://www.analog.com/en/analog-dialogue/articles/deeper-look-into-difference-amplifiers.html