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Original question:

ADS1256: How to get correct voltage reference

ADS1256: Vref circuit for ADS1256

MIrtcho Maglijanov
Prodigy 120 points
Part Number: ADS1256
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 

  • 0
    TI__Mastermind 35141 points

    Hi Mirtcho,

    The reference inputs on the ADC are switched-capacitor inputs which require a buffered reference source. Therefore, you would either need to add a buffer on the output of the reference or switch to a buffered reference, such as the REF6025.

    The OPA350 used on the ADS1256 EVM is a good amplifier, but its DC performance is not the best for very high precision applications. You could use a composite reference drive circuit to get high drive capability with excellent DC performance. An example of such a circuit is shown on page 19 in this reference design using a higher-speed SAR ADC, the ADS8881, though the same design principles apply. In fact, this section discusses the drawbacks of using the OPA350 and why the composite solution is better. You can also look at page 50 of the ADS9110 datasheet for another example of a composite reference drive circuit. While these are SAR ADC examples, delta-sigma ADCs can require reference drive circuitry as evident by the ADS1256. However, many delta-sigma ADCs include reference buffers in the device, so something like the REF5025 could be used in that case. An example of this would be the next generation ADS1256, which is the ADS1260. This has both VREF buffers as well as internal reference. This might be another ADC to consider for you application.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 120 points

    Hello Bryan,

    Thank you very much. Your answer helps me in many way - sigma delta fundamentals, design considerations and knowledge of TI SD ADC product ranges.

    I have used the proposed reference design to calculate my reference buffer circuit. http://www.ti.com/lit/ug/slau515a/slau515a.pdf

    I have two questions related to output RC filter calculations. Both are related to page 15. equation (35) Qref= Iref* Tconv_max

    1. I calculated Iref using ADS1256 datasheet

    Voltage reference impedance@fCLKIN = 7.68MHz= 18.5kΩ

    Worse case would be

    Iref = Uref/Zref = 2,5/18500=13.51mA

    Is this correct?

    2. Tconv_max?   I have no internal ADS1256 knowledge. I think that I have to use clock period of Sigma delta modulator. ADS1256 uses 4-th order Sigma-delta modulator. In My case I have Fclck=7,68MHz

    Fsd-modulator = 7.68MHz/ 4

    Tconv_max = 1/Fsd modulator = 521ns.

    Am I correct?

    Best Regards,

    Mirtcho

  • 0 in reply to Bryan Lizon86
    Prodigy 120 points

    Hi Bryan,

    I've asked you some ADS1256 paramaters in my previous message . The reason is that I wanted to calculate the reference RC filter according to proposed article 

    http://www.ti.com/lit/ug/slau515a/slau515a.pdf

    Can you provide the needed information?

    If not I'm searching for pragmatical solution - can I copy->paste the RC filter from ADS1256 EVM board

    Second option is not preferred because I don't know the design constrains applied to ADS1256 EVM, nor the parameters taken for OPA350

    Thanks in advance,

    Mirtcho

  • 0 in reply to MIrtcho Maglijanov
    TI__Mastermind 35141 points

    Hi Mirtcho,

    My best recommendation would be to use a reference in our REF6xxx series, which includes a reference buffer, to drive the ADS1256. The different options - REF60xx, REF61xx and REF62xx - just offer different temperature drift specifications. If you used the REF6125 for example, the temp drift is 8 ppm/°C from -40°C to +125°C. Section 9.3.4 in the REF6125 datasheet tells you how to size the output capacitor and ESR on the reference to ensure stability. I would start with the values shown in Figure 50 in the REF6125 datasheet. You can then make changes if required to tradeoff between transient performance and noise.

    The more complex solution would involve the REF5025 and the composite buffer, or even REF5025 and a simple buffer like the OPA320. You can see an example of the tradeoffs between the simple buffer (OPA320) and REF6xxx in the ADS127L01 datasheet, which is another 24-bit delta-sigma ADC. Section 9.1.5 specifically talks about these two solutions, and Table 32 compares their outcomes. The block diagram shown in Figure 118 in the ADS127L01 datasheet is a good place to start if you want to use the simple buffer solution, and it provides the values for the passive components. The RC filter used after the VREF is to limit noise, which is why it has a very low cutoff frequency. Also, it is important to size the capacitor on the buffers' output to ensure stability while also being able to provide the necessary charge for current draw on the VREF pin. This would likely to have to be done via Spice simulation, where you would have to model the VREF circuit inside the ADS1256. This is shown in Figure 12 in the ADS1256 if you want to try this in Tina Spice, for example.

    So, the simplest and recommended solution will be the REF6xxx. But you can use a REF + buffer circuit, which will require some simulation on your part. You can see examples of both of these circuits in the ADS127L01 datasheet.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 120 points

    Thanks Bryan,

    Your answer solved my issues.

    Best Regards,

    Mirtcho