Q
The specifications for the AD8230 INA gives typical bias current of 150pA, max1nA at 25degC. Can you tell me how this varies over the specified temperature
range? I cannot find a graph of this or a 'max val over temp' specified.
I have a question also about figure 17 on rev B of the datasheet. It shows how
Average Bias current varies with input CMV for various temperatures. The units
of figure 17 (ds revB) is in uA. Shouldn't this be nA?
A
Since we don’t specify input bias variation over temperature for this part, Ihave very limited data. The good news is that most of this bias current is
related to charging a sampling capacitor at the output, which barely changes
over temperature. This means that the input bias remains below 10nA when
common-mode voltages are close to zero (for split supplies).
However, this does not hold true for common-mode voltages other than zero. The
sensitivity of the bias current to common-mode voltage variations is much
larger than to temperature, which leads me to your second question. Yes, the
plot is correct, the scale is in uA. This is, once again, due to the nature of
the charging of the sampling capacitor, which is reset every cycle. This is why
the behavior looks linear, as it behaves as a switched-capacitor circuit rather
than a high-input impedance gate, like regular amplifier inputs.
In conclusion, you can get to this conclusion from observing figure 17. Notice
how the input bias current lines cross around zero, suggesting very little
variation in input bias over temperature when Vcm=0. The input bias variation
over temperature also increases with Vcm.
