ADS8688: AFE/Differential to Single Ended

Hello Girish,

ADS8688 can only support single-ended input signal. To support differential analog signal, there are a couple of solutions:

1.  Use an instrumentation amplifier or precision amplifier on the load cell channel before the ADS8688A. Please see app notes:

• Circuit for driving a high voltage SAR with an instrumentation amplifier
• Circuit for driving high-voltage SAR ADCs for high voltage, true differential signal acquisition

These app notes were designed for ADS8568 HV ADCs, and they can be used for ADS8688A ADC family as well. Note that the BW of  instrumentation amplifier is usually low.

2.  Use regular HV precision amplifiers to convert differential signal to single-ended signal for the ADC.

3.  Use two channels on the ADS8686S to sample one differential signal, thus you can get the benefits:

• Be able to capture the differential signal.
• Achieve 2~3dB higher SNR since the dynamic range is increased.

ADS8686S has total 16 channels, similar structure as ADS8688A including bipolar direct input with different ranges on each input channel, integrated analog-front-end (AFE) including PGA and ADC driver, single power supply, higher sampling rate 1Msps with 8-to-1 internal multiplexer. Also, this ADS8686S has two ADC cores so that you can use one channel of core 1 and one channel of core 2 to sample your differential analog signal synchronously.

Let me know if you have any questions.

Best regards,

Dale

Thanks for your information Dale,

I will go through the ADS8686S 

Is there any part in sigma-delta type ADC???

which may 8/16 channel with integrated AFE

Hello Girish,

I do not know your detailed requirements in terms of  signal level, data rate etc. However, yes we do have some Delta-Sigma ADCs which support differential input directly, please see ADS131M08 and ADS131E08.

Best regards,

Dale

Hi,

My input signal from the sensor is 4 to 20mA & 0 to 10V both are differential only. I need to convert this differential signal to single ended.

Hi Girish,

I'm not an expert on this AFE device, however it looks like this AFE  can support a differential input and single-ended output, note that the AFE's output is limited by 5V which is its power supply, thus you will have to modify the default +/-10V input range to an unipolar input range on the ADC to match the AFE's output to utilize the maximum dynamic range of ADC.

Best regards,

Dale

Hello Dale Li,

As you told ADS8688 can only support single ended inputs, I'd like to know why do the IC has AIN_X P AIN and _X GND pins. It could be a single ground for all analog inputs in the place of eight input grounds!

Hi Rajesh,

It's related to the internal input structure, also all inputs are independent of each other.

Regards,

Dale

Hello Dale Li,

So, We have to give single ended signals to ADS8688 even though the analog signals are being derived from the same ground.

Hi Rajesh,

The is not related to the same ground plane. The guideline and recommendation in the datasheet should be followed to achieve the performance specified in the datasheet. If you have other questions, please open a new thread, thanks.

Regards,

Dale