ADS124S08: +-24mA measurement and Power supply

Hi Dingli,

Welcome to the forum! See may answers to your questions below.

Best regards,

Bob B

Dingli Xie said:

Part Number: ADS124S08

Hi,

I have to use the ADS124S08 in one Project. One task is to measure +-24mA current over a 100Ohm resisiter. I use +5V to AVDD and link GND to AVSS.

I have few queries related to its analog front-end and negative input measurement as below:

1.On ADS124S08 EVM board Table 14, J13 should be connected to VERF 2.5V to do a single end measure. Can I use a 2.5V biase connected to AINCOM inside ADC instead? [Bob] I'm not totally clear if you are asking the questions relative to a +/- 24mA input or +24mA measurement only.  If the measurement is single-ended then it would be referenced to analog ground.  In this case using the EVM you would connect the ground connection of the current source to J9 pin 3 (common) and the + of the current source to J9 pin 2.  In this case the AINCOM can connect to analog ground (AGND) by setting JP13 to pins 1 and 2.  If the measurement is truly +/-24mA, then you would need to establish a pedestal for the return current set to mid-AVDD supply. The VBIAS does not have good drive capability for sinking and sourcing current due to the switch resistance typically of 350 ohms.  The EVM also has a 4.12k series resistance.  The combination of resistances might cause some measurement issues if there is any leakage in the path.  Your circuit may have similar issues.  Using VREF (either by external reference or by the ADS124S08 REFOUT) can be directly connected giving a solid 2.5V bias on the current source input.  In the case where the source is referenced to analog ground and you want to measure +/- current, then you will most likely need +/- supplies.

2. For Thermocouple measurement, can I use 2.5V biase connected negtive AIN input? Then I do not need biase resister anymore. [Bob] For the thermocouple input you can use the VBIAS to set the proper common-mode for the thermocouple.  The thermocouple is one sensor type where the VBIAS can be used effectively.

3. In reference design TIDA-00550, +-2.5V voltage is uesd while ADS124S08 EVM chooses  +5V to AVDD and GND to AVSS. Which one do you recommond for bipolar input measurement? [Bob] Either configuration will work well.  The advantage of using bipolar supplies is that you can connect your voltage or current source common directly to analog ground (AGND).  For the EVM we simplified the power supply options by only allowing +5V or +3.3V for the analog supply.  You can choose whatever supply option will work best in your design.  If your design will have the capability for +/- supplies, then it probably makes the most sense to use +/-2.5V supplies for AVDD/AVSS.  Also, if there is a possibility that the voltages and currents measured are not isolated from AGND, then this would be another reason to use +/-2.5V supplies.

Regards

Dingli Xie