Why voltage references bite
Q. Why does my voltage reference have nowhere near the accuracy guaranteed on the data sheet?
A. Because you’re being unkind to it. If you’re unkind to a voltage reference it will bite you.
There are three common unkindnesses that can cause your problem. Insufficient headroom, incorrect loading, and reversed output current. The first two are usually mentioned on the data sheet and should be easy to avoid, but the third is rarely mentioned and causes problems which may be hard to diagnose.
Most voltage references have input, output and ground terminals – the output terminal is maintained at a precise voltage above ground for a wide range of input voltages and load currents. But if the difference between the input and output voltage is too small the output voltage precision is degraded. Some devices actually specify this – full accuracy is maintained above supply voltage X, while proper operation, but lower accuracy,
occurs above supply voltage X – but more devices define the range of supplies for full performance but do actually work, but not so well, at slightly lower voltages. It is essential to work in the full precision region to obtain the specified accuracy, but it is well to understand that a reduced supply may cause reduced accuracy, and to check the supply if your reference is inaccurate.
Most voltage references have current-limited outputs so that they will not be damaged by short-circuits. If called upon to deliver too much current their output voltage will drop – and the effect will start well below the point at which the device goes into full current limiting. Check the data sheet both for maximum load current, and for the output current at which the accuracy starts to fall (this is often on a graph).
Another way of loading a voltage reference incorrectly is to use incorrect capacitive loading – many, or even most, voltage references are stable with any capacitive load but some, especially some low drop-out (LDO1 ) types, may oscillate with too much or too little load capacitance (or even with either!) If this happens the output voltage will cease to be correctly regulated. RTFDS2 or experiment to ensure that the range of capacitance a voltage reference encounters in your application does not cause such oscillation – and remember that in a complex system several sub-systems may share a reference, and you may not be responsible for designing all of them.
Next: Bitten again
I was bitten by the third problem myself a few weeks ago. I was designing two very simple low-power battery management systems, the equations defining the resistors in the voltage sensing part of the systems were simple, but when I built them neither worked at anywhere near the correct voltage. (See my EDN article entitled: Two simple secondary battery circuits.)
It took me a couple of days before I realised that the voltage reference in both of these devices were driving the non-inverting input of an op-amp with positive feedback configured as a comparator3 with defined hysteresis. When the op-amp output was high the feedback resistor was driving about 4μA back into the voltage reference output.
I was using ADR291 and ADR292 references and the "simplified schematic" on their data sheet showed the output being driven by an op-amp-like structure. Op-amps can both source and sink current at their outputs and I had sub-consciously assumed that these references would, too. Not so! A reverse current of under 2 μ A is enough to send the output voltage much higher.
The data sheet gives no explicit warning of the problem at all. Load regulation is defined with output currents of 0-5mA, which suggests that large reverse currents (tens or hundreds of μA!) might present a problem but there is nothing to suggest that very small reverse currents might not safely flow in the resistor chain R1, R2 & R3 shown on the simplified schematic.
Once you are aware of this problem it is easily avoided. Many voltage references will sink as well as source current and if the data sheet defines the output voltage for output currents of ±XmA then you may be sure that this is so. Alternatively, if you know that current will flow into the reference output terminal then ground that terminal with a resistor small enough to sink whatever current you expect. This ensures that the current in the reference output is always out of the device and the problem is solved.
James Bryant [firstname.lastname@example.org] has been a European applications manager with Analog Devices since 1982. He holds a degree in physics and philosophy from the University of Leeds. He is also C.Eng., Eur. Eng., MIEE, and an FBIS. In addition to his passion for engineering, Bryant is a radio ham and holds the call sign G4CLF.
1 A low drop out (LDO) reference (or linear regulator) is one which uses an output stage that allows the input voltage to go very close (a few hundred mV or even less) to the regulated output voltage without loss of output voltage accuracy.
2 RTFDS = Read The Friendly Data Sheet.
3 Be careful when using op-amps as comparators, there are potential described in RAQ11 and its expansion. The op-amps I used in both these designs were carefully chosen to avoid such problems.
This article first appeared on EE Times’ Planet Analog website.
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