We use ADC in our regular circuits. The purpose of ADC is to convert the Analog value to a digital value which can be read by the devices like microcontroller. The sensor output and ADC input are not always at same level. To bring down the sensor level read to compatible level at the ADC input, we commonly use resistive divider circuit. The reason to use resistive divider circuit is that the current is very low as the input impedance of the ADC is very high. For example, if we take the case of ADS1114IDGSR, the impedance at the input of the Analog input pin is as below from it's datasheet.
For example, if the input voltage is 3.3V, the current at the input is,
I = 3.3/6M = 0.55uA
This is very less current to create a voltage drop with a resistive divider circuit.
- As the input current is very less, we can have a resistors in the Kilo ohms range
- In case, the sensor has high impedance at the output, it is good to use low value resistors, in the Ohms to low kilo ohms range.
- In case of impedance mismatch, it is preferred to use a buffer in between the sensor output and ADC input
- Add a low pass filter to filter out the noise at the input of the ADC. The sensor signals might get affected by noise and having a LPF at ADC input is always a good option. The cut-off of the LPF depends on the sensor input frequency.
- Add series resistor (10–100 Ω) near ADC pin to isolate from Capacitive loads and this also limit clamp currents. We used 33 ohms resistor in the above circuit
- Use Schottky clamps to rails only if needed. this is for protection at the input. Boards with compliance requirements must have these.
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