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OPA335 Input Inpedance as Unity-Gain Amplifier

Mikael Ducharme
Prodigy 40 points
Other Parts Discussed in Thread: OPA335, TINA-TI

Hi,

I'm trying to use OPA335 as a unity gain buffer. I use a +/- 2.5V supply and my application (EEG active electrodes) makes the AZA, CMRR and 1/f noise features very appealing. High input impedance is also mentionned in the datasheet, but no specific values are listed. My application would typically require >1GOhm input impedance for frequencies under 100 Hz.

I'm trying to find out the input impedance using a 1A current generator in TINA-TI. With an AC analysis, the input impedance I get is the 220K resistor, so there must be something that I'm missing regarding the unity gain configuration of this amplifier. Otherwise, it would imply the OPA335 input impedance is negligeable to the 220k resistor.

Also, is there any particular reason why the input impedance is never specified in the datasheet?

Best regards,

  • 0
    TI__Guru* 93105 points

    Hello Mikael,

    Although the OPA335 is an auto-zero input operational amplifier the outside world looks into the very high impedance gates of CMOS transistors and reverse biased ESD cells. The input bias current is very low, typically 70 pA and the resistive (R) real part of input impedance (Z) is extremely high; typically many Gigaohms for this kind of input design. It is difficult to measure and because it is so high datasheets sometimes don't list it. What is listed in the datasheet are the the common-mode (5 pF) and differential mode (1 pF) input capacitances. These might be more important in applications where the input capacitance plays a role in the overall circuit performance such as transimpedance and charge amplifiers. 

    I checked the OPA335 TINA model to determine if it models input impedance correctly. The model description internal to the model mentions that it models input bias current and the input capacitances are modeled, but there is no mention about input impedance. My model testing used a change of the input voltage, divider by the change in input bias current to determine Zin. It produced a number of about 13 Megohm which is too low; therefore, the model didn't emphasize the parameter.

    The OPA335 should be usable in many ECG applications. Here is an example where it has been applied in ECG:

    Regards, Thomas

    PA - Linear Applications Engineering