MCP3204/MCP3208 Input Resistance Question

Spere

The current circuit will be used to monitor 2 separate (common ground) battery banks in a vehicle ("12" vdc system).

I'm having a heck of a time making sure I have the optimal input resistance setup for the MCP3204.

My main question is: what are the optimal sized resistors to use for 2 voltage dividers (same voltage ranges) in order to give the proper RC circuit timing with the MCP320x internal sampling capacitor.

From what I understand:
On the fast end, the input resistance needs to be at or under 1K ohms (Including the MCP's internal 1K sampling switch) in order to maintain the max 2 mHz speed.
On the slow end, the MCP becomes inaccurate below 10 kHz due to capacitor leakage/discharge.

I'm happy anywhere in between.

I am also attempting to sync up my time constants a bit on all 4 channels of the MCP3204. 2 channels are used for the voltages, one is used for current, and the last is used for user input switches.

I'm leaning toward replacing the 1k resistors on the ground side of the voltage dividers with 10k resistors and adjusting the R1 resistors accordingly (2.37k becomes 23.7k).
Then adding a 10k resistor inline with the current sensor.

If I did my calculations correctly, this will give me an overall resistance of 7.032k as seen by the ADC input pin.

Here is the schematic and some items from the data sheet.
MCP3204 Resistor Sizing - Schematic - v1.0 .pdf
MCP3204 Resistor Sizing - Notes - v1.0 .pdf

The full Data Sheet can be found here:
http://ww1.microchip.com/downloads/en/devicedoc/21298c.pdf

Thanks!
Scott

PropGuy2

I have tried both the 1K and 10K ground side resistors, and found that both give accurate readings. In the end, I decided to use the 10K ground side resistor just to save current draw on the voltage divider circuit. I do not use the current limiting resistor going into the ADC input pin since the current is already very low. I do however use a precision MCP1525, 2.5VDC voltage reference ic chip on pin (Vref) of the MCP3204/08. This improves the accuracy of your readings significantly. FYI the MCP1525 needs a 1uF cap on the output pin to work correctly. I have included a MCP3208 code snippet for reference.

kwinn

These points from page 13 & 14 of the data sheets are the critical items for accurate measurements.

For the A/D converter to meet specification, the charge holding capacitor (CSAMPLE) must be given enough time to acquire a 12-bit accurate voltage level during the 1.5 clock cycle sampling period.

This diagram illustrates that the source impedance (RS) adds to the internal sampling switch (RSS) impedance, directly effecting the time that is required to charge the capacitor (Csample). Consequently, larger source impedances increase the offset, gain and integral
linearity errors of the conversion (see Figure 4-2).

A lower sampling rate will allow the sampling capacitor to charge more fully, and placing a small capacitor (0.1 or 0.01uF) across the 10K resistor between gnd and the input pin will reduce the effective input resistance and help reduce any noise on the input.