TLC251 Input Impedance

Hi Smiffy,

The vagueness of the extremely high input impedance is due to the difficulty of measuring it. The TLC251 input path consists of the input ESD cell and the gate of a P-Channel MOSFET. The input bias current is comprised of their leakage currents. The actual input impedance is determined by how the leakage current changes with a change in voltage at the input.

Let's say for example the room temperature input bias current was 1 pA, and it changes 1 pA with a 2 V change in input voltage. Taking the 2 V change and dividing it by the 1 pA change results in an input impedance of 2e12 Ohms (2 tera-ohms). I don't know if these levels of change are even close for the TLC251, but my example illustrates how the input impedance can be extremely high. I sometimes see CMOS operational amplifiers having input impedances listed in the range of 10¹² to 10¹⁵ Ohms, which I would classify as extremely high.

Do note that input leakage current, and hence input bias current, are a function of temperature and I would expect the input impedance to be affected by the temperature as well.

Regards, Thomas

PA - Linear Applications Engineering

Hi Smiffy,

Here's my thoughts about your discussion:

Would this part be suitable in the input stage of an electrometer, or would the leakage of the ESD cell make it unsuitable?

The TLC251 input bias current can be as high as 60 pA, at 25 C, and even higher at high temperatures. This is probably higher than can be tolerated in most electrometer applications.

Discrete FET designs I've looked at have leakage figures in the fA range, and op amps I've seen recommended are both relatively expensive and easily destroyed (I have this on good authority from someone who has killed a good few!) Happy to carry on as I a (discrete FET followed by a general purpose bipolar op amp) but if this low-cost component, with it's protection network, were up for the job could be interesting to try it out.

I can't speak regarding the fragility of operational amplifiers intended for electrometer applications. I just haven't seen reports about it. I do know that our Silicon Valley division, formerly National Semiconductor, does produce some very low input bias current operational amplifier products that are quite suitable for electrometers. The LMC6001 (25 fA) and LMP7221 (3 fA) are single-supply amplifiers and popular for such applications.

We produce the OPA129 (100 fA max) operational amplifier and an easily applied instrumentation amplifier, the INA116 (3 fA). This latter device has input voltage protection built in making it less worrisome where an input over-voltage might be a problem.  Both of these devices are intended to use dual supplies (+/-V) which is something that must be considered.

Regards, Thomas

PA - Linear Applications Engineering