AD9057: BIAS out grounding and drive current

Document created by analog-archivist Employee on Feb 23, 2016
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In a prototype construction we are using an AD9057 A/D converter and my
question is can the BIAS OUT (pin 6) be grounded ? Or will this degrade the
performance of the A/D converter (vref out/in)?

I also would like to know if the internal reference (page 7 in the data
sheet)is capable of supplying 300uA of drive current over the full temperature
range -40 to +85°C.
Or will the drive current ability be decreased at -40°C


Looking at the ADC input, the Bias out pin is connected to the REF IN pin via a
1k resistor. Therefore the current drain incurred by connecting BIAS OUT to GND
is 2.5mA.
The on-chip reference can only supply around 300uA so you should not connect
the BIAS OUT to GND if you are using the on-chip reference. If you are using an
external reference, you need to evaluate the loading effect of the 1k to
ground. You also need to consider the heat dissipated by the 1k resistor and
the increase in power consumption. If the BIAS OUT pin is unused, it's probably
better to connect it to REF IN so that there is no voltage drop across the

Looking at the section of the datasheet which talks about the reference, the
figure of 300uA is mentioned informally. It should be interpreted in the same
way as a typical spec i.e. you should not rely on it if you absolutely need
300uA minimum. The 300uA is not guaranteed at 25degC or over temperature.