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I am making an ADC circuit using ADS1115. It is 4 channels which I will be using to measure 2 signals in differential mode. On the Layout and Examples section of the datasheet it says:

Analog inputs with differential connections must have a capacitor placed differentially across the inputs.

In the layout example it basically is following this circuit:

enter image description here

What are these components for? It is in the guidelines so I assume they are important components. And most importantly how do I calculate their values?

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  • \$\begingroup\$ Technically, they are ADC input capacitors and resistors. \$\endgroup\$
    – DKNguyen
    Feb 14, 2020 at 22:10
  • \$\begingroup\$ Why are your inputs labeled 'AXout'? Why is your schematic reading right to left? \$\endgroup\$
    – Transistor
    Feb 14, 2020 at 22:18
  • \$\begingroup\$ @Transistor i meant it be connected to a breakOUT header hence the out, i apologize if it was misleading \$\endgroup\$
    – Jake quin
    Feb 14, 2020 at 22:35

2 Answers 2

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Take a look at the example circuit in figure 44 on page 36 of the datasheet: enter image description here

There's your mysterious capacitor across the differential inputs again.

Now scroll down to page 38 and have a look at section 10.2.2.6 "First-order RC Filter Considerations"

Although the device digital filter attenuates high-frequency noise, use a first order low-pass RC filter at the ADC inputs to further reject out-of-bandwidth noise and avoid aliasing. A differential low-pass RC filter formed by R5, R6, and the differential capacitor CDIFF sets the –3-dB cutoff frequency, fC, given by Equation 16. These filter resistors produce a voltage drop because of the input currents flowing into and out of the ADC. This voltage drop could contribute to an additional gain error. Limit the filter resistor values to below 1 kΩ.

fC = 1 / [2π · (R5 + R6) · CDIFF] (16)

Two common-mode filter capacitors (CCM1 and CCM2) are also added to offer attenuation of high-frequency, common-mode noise components. Select a differential capacitor, CDIFF, that is at least an order of magnitude (10x) larger than these common-mode capacitors because mismatches in these common-mode capacitors can convert common-mode noise into differential noise.

The two resistors R5 and R6 together with your capacitor form a low pass filter to remove high frequency noise from your signal. You need that to help the built in antialiasing filter do its job.

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  • \$\begingroup\$ In the table just below that Table 11, are values that will generally work for most purposes, but if i want to customize it a little bit to my purpose what should i look for first? It would seem best to find Fc first, but how do i know that i want a -3db cutoff? \$\endgroup\$
    – Jake quin
    Feb 14, 2020 at 22:23
  • \$\begingroup\$ The highest frequency of your signal and the sampling rate. Stay below half the sampling rate, and above twice the highest frequency you are interested in. \$\endgroup\$
    – JRE
    Feb 14, 2020 at 22:25
  • \$\begingroup\$ My traget frequency is 40 - 70 Hz, isnt using the maximum sampling rate better rather than using half? \$\endgroup\$
    – Jake quin
    Feb 14, 2020 at 22:29
  • \$\begingroup\$ Your sampling rate always has to be at least twice the highest frequency that you expect in your signal. See the Wikipedia page on the Nyquist-Shannon sampling theorem. \$\endgroup\$
    – JRE
    Feb 14, 2020 at 22:33
  • \$\begingroup\$ oh i think i just misunderstood. of course it should be higher. knowing my cutoff frequency, there would still be 2 variables missing R and C so i have to assume a value for one of them ? Which would be a better/safe variable to assume \$\endgroup\$
    – Jake quin
    Feb 14, 2020 at 22:44
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Here's the relevant part of the datasheet that you linked:

enter image description here

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