Other Parts Discussed in Thread: REF5025, OPA388, OPA625, OPA350, ADS1255, ADS1259, PGA280, ADS9110, ADS8881, ADS1260, REF6125, OPA320, ADS127L01
Hello,
I have few quiestions related to ADS1256 Vref driving circuit.
1. What are the ADS1256 requirements for Vref drive circuit?
2. Can I use REF5025 without buffer?
3. How much is the ADS1256 capacitive load in picofarads?
4. Can I use use single OPA388 as buffer, or do I need multi-loop buffers with OPA388 ->OPA625/OPA350
The reason why I ask this questions is that I the information from different TI sources speaks against ech other.
- ADS1256 datasheet page 17 "The load presented by the switched capacitor can be modeled with an effective impedance (Zeff) of 18.5kΩ for fCLKIN = 7.68MHz."
- ADS1256 page 28 "For voltage references not suited for driving the ADS1255/6 directly (for example, high output impedance references or resistive voltage dividers), use the recommended buffer circuit shown in Figure 26." -> buffer circuit using OPA350
- REF5025 datasheet
- TI precision Lab series advice REF60xx instead of REF50xx to drive highly capacitive SAR ADCs. But it seems not needed in case of sigma delta
https://training.ti.com/ti-precision-labs-adcs-overview-reference-drive-topologies
- ADS1256 EVM uses REF5025 buffered with OPA350. Why? is REF5025 buffer capability not enough?
http://www.ti.com/lit/ug/sbau090e/sbau090e.pdf
- A sow some other ADS1256 boards that uses OPA350 too. The result is that this simple buffer circuit ruins the low noise and low temperature coefficient reference circuit . No wonder that the system performance is somewhere in 12 bits range.
==============
My system requirements
- part of ATE system
- DC voltage measurements ranges +-100mV ...........+-10V.
Design notes
- ADS1256 - use only one differential channel. Chosen doe to logistic issues - ADS1256 is chaper than ADS1259
- PGA280