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,
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]
Marek LisSr Application EngineerPrecision Analog - Burr-Brown ProductsTexas Instruments - Tucson
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?
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.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with respect to these materials. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.