Dear Sir
1. When I use ADS8860 ,should I have to nee Voltage reference with ADS8660 or I can use low noise ldo with ADS8660
2. Do you have document to explain why need low output impedance buffer drive ??
3. If I use 2pcs ADS8860 on the board ,should I put 2 pcs REF60xx or I can put 1pcs REF60xx on the board
Thanks
Hi Kai,
-In general, LDO's will tend to present larger voltage drift and higher noise than a dedicated precision voltage reference. The ADS8860 is a 16-Bit SAR ADC, were the accuracy and precision of the conversions are a direct function of the voltage reference. Therefore, a dedicated low noise, low drift, precision reference is recommended.
-If you plan to use two ADS8860s at full data rate of 1MSPS, the recommendation is to place separate 10uF bypass capacitor for each ADC in close proximity of the REF pin and use separate reference buffer circuits for each. If you plan to use the rEF60xx with the ADS8860 at full data rate, you will require one REF60xx for each ADS8860.
- What is the maximum sampling rate required on your application?
The ADS8860 is a single-ended input SAR ADC, however, if differential inputs are acceptable in your application, we also offer the ADS8920 (fully differential input SAR) which incorporates the reference buffer in the device. You could consider using (2) ADS8920s with a single REF50xx reference. Since the ADS8920 has a built-in reference buffer, the REF50xx can be connected to (2) ADS8920s in the configuration show below:
The reference input of the SAR ADC presents a dynamic load to the reference source and instantaneous current transients are present as the internal capacitor array is charged during the conversions process. During one single conversion, the reference voltage is sampled (several times) and most of the instantaneous charge is provided by the REF bypass capacitor, however, the reference driver circuit (reference buffer) needs to completely re-charge the bypass REF capacitor between conversions. If the reference driver circuit is not able to fully re-charge the reference bypass capacitor after the conversion, the reference voltage will start drooping after a few conversions and errors in conversions will occur.
Below are a few documents that discuss the reference input of the SAR ADC:
http://www.ti.com/lit/an/slyt339/slyt339.pdf
http://www.ti.com/lit/wp/slyy097/slyy097.pdf
https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/07/25/sar-adc-pcb-layout-the-reference-path
https://training.ti.com/how-set-tis-ref6025-voltage-reference-performance-demonstration-kit
Let me know if you have any questions,
Thank you and Best Regards,
Luis
