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I'm looking at the REF30xx series of precision voltage references from TI. The datasheet lists a maximum output current of 25mA. Every specification and figure on the datasheet (e.g. load regulation on pages 3 and 5) shows only positive output current (0-25mA).

This concerns me, because datasheets for other devices such as op-amps tend to have bipolar current ranges (-25 to 25 mA) and figures either have an axis that goes negative for output current or else shows absolute output current (using +/- mA).

Can I expect these voltage reference ICs to operate correctly in a circuit that requires them to sink as well as source current?

If not, can someone recommend a buffer circuit that won't ruin the low-noise characteristics of the reference voltage?

Ben Voigt
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  • How are you using your reference? How much current do you need to source and sink? If you only need to sink current, or if your needs in the sourcing direction are fairly small, you could use a shunt reference instead of a series reference. If you need both sourcing and sinking, then a buffer like you're asking for could be the right approach. – The Photon Sep 11 '12 at 23:23
  • I'm using it for a virtual ground, to keep an inverting amplifier elsewhere in the circuit ([Figure 18 here](http://www.analog.com/static/imported-files/tutorials/MT-090.pdf)) from going negative on the output. Both input and output signal will swing above and below the virtual ground, so the reference needs to source and sink current. The application circuit at the end of the TI datasheet shows it being used for Vref on an ADC. Doesn't that require sinking current? – Ben Voigt Sep 11 '12 at 23:36
  • Depends on the ADC. I would expect that most ADCs would require only sourcing, because the reference voltage is usually at the maximum value for the conversion. But you'd want to check each ADC's data sheet. – The Photon Sep 11 '12 at 23:38
  • @ThePhoton: So you interpret the documentation to mean that this chip cannot sink current? – Ben Voigt Sep 11 '12 at 23:39
  • Yes. It's made explicit in the load regulation spec on pg. 3. Load regulation is only guaranteed for 0 < I_load < 25 mA. – The Photon Sep 11 '12 at 23:43

1 Answers1

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Most references are designed either to source current (series references) or sink current (shunt references), not both. For your part, this characteristic is made explicit in the load regulation spec on pg. 3. Load regulation is only guaranteed for 0 < Iload < 25 mA.

If your current sinking requirements are fairly small, you consider could using some pre-loading (e.g. add an additional load that ensures the reference is always sourcing current even though the "real" load is also sourcing current).

If the current you need to source/sink is too great, or if the power efficiency for a pre-load circuit is not acceptable, then you could consider using a buffer. The buffer amplifier will need to have low thermal drift and low offset voltage (e.g. comparable or less than the allowed drift/error in your reference voltage). Of course you also want low noise and gain-of-1 stability.

You might end up using an instrumentation amp instead of an op-amp to get these characteristics. For example, the AD620 is kind of the "old standby" when it comes to low drift and low offset voltage amplifiers. If you haven't used an in-amp before, be sure to read the data sheet thoroughly before building your circuit. You don't just hook them up like they are op-amps.

The Photon
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  • So we decided to use an [ADR440](http://www.analog.com/en/special-linear-functions/voltage-references/adr440/products/product.html) which explicitly supports current sinking as well as sourcing. – Ben Voigt Sep 12 '12 at 16:12