ADS8689: schematic check

Hi Yolanda

1. For the application of 40V Input, should the circuit be changed to the following settings?

2. For applications that require 20V~40V INPUT, how should the formulas corresponding to the input resistance and capacitance be calculated?

Hello Gareth, 

Yes, if you follow this reference document the image you shared from (Circuit to increase input range on an integrated analog front end (AFE) SAR ADC) it goes through some of the calculations of how those components were selected. 

This system works because these devices (ADS8689 and ADS8598H) both have an AFE with a 1MΩ input impedance, so by making a voltage divider with the impedance the ADC inputs actually see the ±10V. This would be one of the simpler approaches to achieve ±40V input. If that input voltage changed all that would need to be adjusted would be the Rext in the diagram you shared (or the reference shared). 

Would this simpler approach be the most useful in your system? Or if you wouldn't mind sharing a bit more about your systems needs I can help a bit more. Besides needing to measure and input range of ±20-40V, what kind sampling speed, resolution, channel count?  

Other useful references that share additional methods of scaling down the input voltage with additional circuitry are High-Voltage Signal Conditioning for Low-Voltage ADCs, Circuit for driving high-voltage SAR ADCs for high voltage, true differential signal acquisition, High-voltage battery monitor circuit: ±20V, 0–10kHz, 18-bit fully differential

Best regards, 

Yolanda

Hi Yolanda,

For the figure in below you modified, the setting would be suitable for single-ended +/-40V input?

Thanks!

Hi Yolanda,

Also, with below design when shorted inputs together to GND and with some floating value as 0.0033.0013.

How to do the calibration?

Also send a mail to you to help check the schematic and layout, thanks!

Hello Zoe,

In the reference document shared it gives an example of a two-point calibration that can be used for that system. 

Also, looking forward for schematic, thank you. 

Best regards, 

Yolanda

Hi Yolanda,

Already sent you a mail and please help check~

Could you also help provide a calibration excel file and any training video for the calibration?

Thank you!

Hello Zoe, 

The document Circuit to increase input range on an integrated analog front end (AFE) SAR ADC explains how to perform a two-point calibration. 

For training videos, the Understanding and calibrating the offset and gain for ADC systems video in the Precision labs series: Analog-to-digital converters (ADCs) should be helpful. This also covers the two-point calibration. 

Best regards, 

Yolanda

Hi Yolanda,

Could you take a look for the mail?

Thank you!

Hi Yolanda,

Please help review the schematic and layout soon.

Main is B version and also point out the different for A/B version.

Please help check B version schematic/layout is good for single-ended +/-40Vin or not and any need to adjust?

Thank you!