• TI Thinks Resolved

Hi,

I'm planning on using ADS1282 on an Eval board, in bipolar mode for Gain Cal, I plan to use +/-2.5V on  VREFP/N & +/- 2.5V on AVDD/AVSS. (Vref = VREFP – VREFN) which should be Vref = 2.5 - (-2.5) =5V with PGA =1, is that correct ?

In Bipolar mode with a 5V Vref (as above) for gain cal must supply input signal pins should be +1.25V to INP and - 1.25V to INN  that correct? +/-Vref/(2*1) = 2.5/2 = 1.25V

in Unipolar I plan to have 5V on VREFP & 0V on VREFN, is that ok?

Thanks, Chris.

• Hi Chris,

Welcome to the TI E2E Forums!

Correct...The ADS1282 will take a 5V (differential) reference voltage and measure input signals up to +/- 2.5V (differential). Therefore, with a 5V reference, your full-scale input signal for gain calibration will be 2.5V.

VREFN is typically biased to AVSS, so in bipolar mode AVSS = VREFN = -2.5V; while in unipolar mode AVSS = VREFN = 0V.

Best regards,
Chris Hall

TI Precision Data Converters| Data Converters Learning Center | Selection Guide

• In reply to Christopher Hall:

Hi Chris,

Thanks for confirming my differential set-up is correct.

For the Unipolar config with AVDD = VREFP = 5V, AVSS = VREFN = 0V, would AIP = 2.5V & AIN = 0V, is that correct?

Thanks, Chris.

• In reply to Chris Sanderson:

Hi Chris,

There is an "absolute input range" requirement on the ADS1282 that says that AINP and AINN should not exceed (AVDD - 1.25V) or drop below (AVSS + 0.7V). ...This is another way of specifying an input common-mode range requirement.

The differential 2.5V is okay, but the common-mode voltage would out of specification if AINN < 0.7V.

I would recommend setting AINP = 3.475V, and AINN = 0.975V to bias the 2.5V differential signal to the center of the PGA's input common-mode range; however, any 2.5V differential voltage that meets the "absolute input range" requirement will work.

Best regards,
Chris Hall

TI Precision Data Converters| Data Converters Learning Center | Selection Guide

• In reply to Christopher Hall:

Hi Chris,

So for Unipolar I plan to use +5V on AVDD/VREFP & 0V on AVSS/VREFN and for the input pins, AINP = 3.475V, and AINN = 0.975V.

Thanks, Chris.

• In reply to Chris Sanderson:

Hi Chris,

That is correct.

Best regards,
Chris Hall

TI Precision Data Converters| Data Converters Learning Center | Selection Guide

• In reply to Christopher Hall:

Thanks Chris.

One last question hopefully.

For the Unipolar input pins, AINP = 3.475V, and AINN = 0.975V can we change this to AINN = 3.475V, and AINP = 0.975V? is that a concern.

Thanks, Chris.

• In reply to Chris Sanderson:

Hi Chris,

It's not a problem to swap AINP and AINN, but you'll have to manually perform the calibration as outlined in section 9.30 of the datasheet. When computing equation 11, the negative full-scale code will be your expected output code.

If you send the GANCAL command the device is going to expect a positive full-scale signal. Running GANCAL with a -2.5V differential input may produce some odd results in the FSC[2:0] registers.

Best regards,
Chris Hall