Filter position for delta-sigma ADC

Question

I am designing a ADC module for torque-based motor control application.

• The input source is Wheatstone bridge load cell with excitation voltage of 3 V.
• The loop time of controller would be around 300 Hz.
• Since the torque source also have bandwidth lower than 300 Hz, the frequency of interest would be about <300 Hz.
• 24-bits Delta-sigma ADC MCP3561 would be operated with output data rate of 300 Hz.
• The excitation voltage and ADC reference voltage is supplied by REF3330 voltage reference.
• I have noticed that, since the impedance of load cell is quite low(<300 Ohm), current drained from voltage reference is quite significant(>10 mA, REF3330 output current: 5 mA, REF3030 output current: 25 mA).
• Now I noticed that voltage-follower (or buffer, ADC driver) is needed to add large impedance and reduce current.
• Considering the noise characteristic, rail-to-rail feature, offset voltage, production issue and etc., the op amp OPA376(7.8 nV/sqrt Hz) has been chosen for ADC driving and possibly active filtering.

And here are my questions:

In this kind of application, where should the passive low pass filter be placed? In some design it is placed before the buffer:

BRIDGE SENSOR - PASSIVE LPF - BUFFER - ADC

In another, after the buffer:

BRIDGE SENSOR - BUFFER - PASSIVE LPF - ADC

I believe in the former case the op amp will not suffer for driving quite large capacitive load (~ a few uF in this case) but have some disadvantages on voltage drop before the signal enters the buffer.

On the contrary, in the latter case, a quite clean signal comes in to the module without the possibility of any other load distorting the signal. However, a large capacitor after the buffer might be a problem for the op amp and settling time must be dealt with carefully.

If it is okay to drive a ADC directly with voltage follower, I guess

BRIDGE SENSOR - BUFFER - PASSIVE LPF - BUFFER - ADC

could also be a solution.

Or is there any other superior active filter based solution with similar complexity?

Answer 1

I would recommend:

Bridge Sensor -> Buffer -> Passive LPF -> Buffer [Maybe] -> ADC

How I decided the order is based on input and output impedance.

Circuit	Input Impedance	Output Impedance
Bridge Sensor	NA	Medium
Buffer	Large	Small
Passive LPF	Medium	Medium
ADC	Varies (I'd say about Medium for your Delta-Sigma ADC)	NA

Generally you want to feed circuits with a low output impedance, and you want to load circuits with a high input impedance. Below is an example of why.

In circuit 1, the LPF is loading V1's output impedance R1. Because of that, the 3dB frequency has shifted by 10%. In circuit 2, a buffer has a large (ideally infinite) input impedance, so the source resistance R3 has little/no effect. The buffer has a small (ideally zero) output impedance, so the filter does not load it, and the theoretical 3dB frequency is as expected.

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Looking at your suggestions:

BRIDGE SENSOR - PASSIVE LPF - BUFFER - ADC

^ Passive LPF loads the Bridge sensor. Would also need a differential filter.

BRIDGE SENSOR - BUFFER - PASSIVE LPF - ADC

^ Probably ok with a delta-sigma ADC (relatively high input impedance). If you were using a SAR ADC and the Passive LPF have a high-ish resistor I'd stick a buffer in-between LPF and ADC.

BRIDGE SENSOR - BUFFER - PASSIVE LPF - BUFFER - ADC

^ Most conservative option. Again, second buffer may or may not be needed depending on ADC and LPF.

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A more complicated, but more compact version could be:

Bridge Sensor -> Differential Active LPF -> [Maybe Anti-Aliasing Filter] -> ADC

Look up Differential Active LPF for implementations.