Hello,
I'm a bit confused regarding input bias current specifications for the OPA727 op-amp. Referring to the datasheet, the "ELECTRICAL CHARACTERISTICS" specify 85pA typ and 500pA max @ 25C, what common-mode voltage is this specified at?
Also, I have a few questions regarding figure 7. "input bias current vs common-mode voltage":
1) The valid common-mode input range is specified at V- to V+-2.5V or -6V to 3.5V in this case, is there something special about the test performed in Figure 7 that lets it extend past the valid range?
2) The region of bias current when the common-mode voltage is 0V has a very small temperature coefficient as per the graph in Figure 7, would this cross-over region always be at the halfway point of the total supply voltage? For instance, if I used V+=6V and V-=-4V, would the cross-over region be at 1V?
Thank you very much for your time,
Tim Hoeppner
Tim,
The typical PDS value of IB is measured with Vcm in the middle between the supply voltages. However, I believe there is an error in the PDS graph (Fig 7) showing IB to be highly dependent on Vcm - typically the slope of IB in CMOS amplifiers is very small since it represents the common-mode input impedance: RINcm = [delta_Vcm/delta_IB] = ~1E+12.
In the OPA727 PDS table below the input impedance is specified to be 1E+11 so 12V change in Vcm should result in only 120pA change of IB.
We have just measured IB of OPA727 for two supply voltages: +/-2V and +/-6V and the results are shown below. Unlike the PDS graph, they do NOT show the unusually large increase in IB as one changes Vcm away from the mid-supply; [IB expressed in pA]
Regards,
Marek LisSr Application EngineerPrecision Analog - Burr-Brown ProductsTexas Instruments - Tucson
Marek,
Your response is very helpful thank you. I'm still wondering why your tests include common-mode voltage which extend past the valid common-mode range? I don't know enough about the internal workings of the op-amp to know what are the side effects are for doing so. The datasheet specifies a common-mode voltage range of (V-) to (V+-2.5).
In the first graph with +/-2V supply, shouldn't you only test from -2V to -0.5V?
And in the second graph with +/-6V supply, shouldn't you only test from -6V to +3.5V?
Thanks,
Tim
We characterize the IB across the entire supply voltage range just to show what will happen if the customer exceeds the input common-mode voltage range, which happens quite often. In the case of the CMOS op amps, like OPA727, IB is not effected since the input current is dominated by the reverse-biased ESD protection diodes. However, in the case of the bipolar transistor op amps, IB increases dramatically once the input common-mode range is violated, which together with the use of high value input resistors may lead to lock-up condition requiring turning-off power supply in order to recover.
Wow, you have been very helpful, thanks.