ADS1285EVM-PDK: Correct Config setting

Hello Shane,

For Gain=1, use the default register settings, as you have shown above.

GAIN2=40h
GAIN1=00h
GAIN0=00h

Same for the offset values, use all default, which should be 00h.

OFFSET2=00h
OFFSET1=00h
OFFSET0=00h

The HPF1 and HPF0 registers set the high-pass corner frequency, but are only used when the high-pass filter (IIR) is enabled by setting the FILTR[1:0]=11b in the CONFIG0 register.  Since you have the filter set to FIR output, FILTR[1:0]=10b, the HPF1 and HPF0 settings have no effect on the output code.  You could set these to any value, but just leave these at the default values of HPF1=03h and HPF0=32h.

Since you want a gain=1, use all default power-up reset values for all registers except the CONFIG registers.  Update CONFIG0 for the desired data rate.  CONFIG1 should be set to input1 (default) for external connections on J12.  Since you are using the EVM with a 4.096V reference, the REF[1:0] bits should be set to 01b as well.  (Looks like you have the CONFIG1 set to 00h, which would produce incorrect readings with a 4.096V reference.)

CONFIG0=00011010b
High power mode
2000sps
Linear phase FIR
FIR output filter

CONFIG1=00001000b
Input1
4.096V reference
PGA gain=1

Regards,
Keith Nicholas
Precision ADC Applications

Using 5V reference, I need to block DC only since the frequencies I am measuring are close to dc up to 20Hz. What is the lowest Fc of the HiPass?

And Thank you for the Unity Gain settings confirmation. If I have differential -6dB into the OPA1632 feeding the ADS1285 EVM with Vocm connected I get -42dB in my spectral analysis

Delete last message re -42dB My mistake!! i get -6dB at 100Hz per input

Need more instruction on DC elimination as close to lowest frequency as possible Thank you 

Hello Shane,

You can manipulate equations 4 and 5 to calculate the high-pass cutoff frequency based on the register value HPF and data rate fdata.

HPF[15:0]=a*2^16

fc = fdata/Pi*ATAN[HPF/(2^17-HPF)]

The lowest corner frequency as a function of the data rate fdata will be with HPF=1.

fc-min = 2.4285*10^-6 * fdata

fc-min(2000) = 2.4285*10^-6*2000 = 0.00486Hz

However, note that using the IIR filter at extremely low frequency will result in a large settling time for any step input voltage change.

For example, a 0V to 1V step input change will require n conversion periods to settle to within a give percentage.

1% settling (x=0.01) with HPF=1 (minimum cutoff frequency) will require:

n=ln(x)/ln(1-HPF/2^16)

n=ln(0.01)/ln(1-1/2^16)=301802 conversion periods. 

At 2000sps, this will take 151 seconds.  This number is similar to an RC analog high-pass filter at this same frequency.

Regards,
Keith