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I have a circuit that is fed into a ADC chip (adc128s022), that I am trying to understand. The sensors are inferred gas sensors. Could you please criticise my analysis.

The way I am reading it:

  1. Diodes between gnd and signal are for clamping the voltage and protection.
  2. Inductor , two caps and 200 ohm resister are a low pass filter.
  3. 10k resister is for further protection.
  4. Op amp is acting to clamp the voltage 0-5V and for impedance matching?
  5. The resister and cap after the buffer are another low pass filter. Is this because the buffer could amplify some small amplitude signal noise?

I am not sure what the point is of the first capacitor, may be to protect the sensor?

I am guessing the different low pass filters before the buffer are for different frequency?

ADC

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    \$\begingroup\$ Just a whine and complaint. ;-) When posting a schematic DO NOT remove the component reference designations--D1, D2, C1, C2, R2, R2, etc., especially for a question like this one where you'd like someone to explain or corroborate each component's purpose. \$\endgroup\$ Mar 30 '20 at 20:56
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This is my interpretation:

  1. Schottky diode D1 provides reverse voltage protection.
  2. TVS diode D2 provides electrical overstress protection against overvoltages on the signal line (e.g., electrostatic discharge (ESD) when connecting or disconnecting the probe).
  3. Inductor L1 is probably an RF choke that blocks high-frequency signals (radio signals, voltage spikes, etc.) from entering or leaving the right-hand side signal conditioning circuitry.
  4. C1 assists L1 in eliminating high frequency noise by shunting high frequency signals to ground.
  5. R1 provides an energy discharge path for capacitors C1 and C2, and possibly helps reduce ringing in L1 and C1.
  6. C2 and R2 form a low-pass filter (LPF). My guess is this filter is placed at the op amp's input for a few reasons. (a) Note that R3 and C3 (at op amp U1's output) also function as a LPF. C2,R2 along with C3,R3 form a second order LPF which provides steeper roll-off above the cutoff frequency, which is a desirable trait for an analog-to-digital (ADC) anti-alias filter. (b) If C2,R2 were moved to the op amp's output and connected in series with C3,R3, the series combination of these two passive filters would adversely attenuate (reduce in amplitude) the signal "ADC", and additional gain might be required from the op amp U1 (<- which would require two additional resistors) to compensate for this attenuation. (c) LPF C2,R2 further reduces any remaining high frequency noise, so that high frequency noise does not adversely affect op amp U1.
  7. Zener diode D3 is the last line of defense against over voltage at the op amp's input, and it ensures the input and output signals are clipped at the Zener voltage (i.e., voltage limiting for the ADC's input).
  8. Op amp U1 is a unity-gain buffer amplifier that (a) (presumably) has high input impedance to minimize current loading on the signal conditioning components and sensor, and (b) provides a very low impedance output signal that reduces ghosting at the ADC. (NB: Passive filter C3,R3 negates somewhat the benefit of answer 8.b.)
  9. C3 and R3 form the second stage of a two-stage, second-order low-pass "anti-aliasing" filter that band limits the signal that's applied to the analog-to-digital converter (ADC) input.
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  • \$\begingroup\$ Thanks for the detailed explanation. Especially the point 6 and 8. Seem like I was on the path (approximately) but missing many finer points. \$\endgroup\$
    – Tommy
    Apr 1 '20 at 12:11
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From your circuit it seems that the signal to be read out, can swing from GND to 24V. From the left to the right, I would say the following:

  1. Schottky diode acts as a reverse voltage protection mechanism.
  2. Bidirectional TVS diode protects against spikes and surges in the signal line.
  3. Inductor, capacitor and 100Ohm resistor form a RLC low pass filter
  4. The 10KOhm resistor along with the Zener diode clamps the maximum voltage, protecting the opamp.
  5. The 10nF will take large dV/dt, which are too fast for the zener to start to operate
  6. Opamp configured as a buffer, decouples the signal to be read out from the ADC by providing respectively a high input and low output impedance
  7. 1kOhm and 10nF form an additional RC low pass filter.
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  • \$\begingroup\$ Thanks. Especially point 5 use of dV/dt. The use of a bit of basic mathematical notation make many things clearer. \$\endgroup\$
    – Tommy
    Apr 1 '20 at 12:13

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