OPA858: OPA858 Spice model

Don't know and I had sort of decided this effect is not usually worth considering. It shows up in the designer sims, but if you start trying to make it a dominant term you can't. I tried to measure it a few times on parts like the OPA656 and by the time I got the resistors large enough to gain it up I was bandlimited by their parasitics or their noise dominated. 

As I understand it, this effect influencing my decision what opamp to use in TIA, with BJT or with FET input.

What do you think about the following TIA:

• OPA858,
• Cin=4.5pF,
• Rf=130K, Cf=60fF.

Simulation shows a bandwidth of 26 MHz, nice phase behaviour, and average RTI noise of about 1 pA/rt(Hz).

Looking at the graph above, the input current noise should become 1 pA/rt(Hz) at 20 MHz.

There are two classical sources of noise in this TIA - thermal noise of Rf and e_n of opamp multiplied by average noise gain of the circuit:

• The 130K resistor makes thermal noise of 0.35pA/rt(Hz)
• The noise gain is high (1+4.5/0.06=76 at its peak, so 38 in average), e_n*NG_avg=2.5n*38=95nV/rt(Hz) at the TIA output, which gives 0.73pA/rt(Hz) at the TIA input
• summing both above gives 0.8pA/rt(Hz).

Simulation shows about 1pA/rt(Hz). Is this because of input current noise rising from the depths, or from other reasons?

So, will they add, on your opinion, and I should expect to measure more than 1pA/rt(Hz), or the model have this effect inside?

Could you please, test the model for i_n vs frequency?

Thank you

Hello, Michael

OK, good news. Hope that numerically it is also done correctly.

Bottom line, it seems to be a very accurate model.

Thank you very much

Nice to talk to you, as always :-)

you bet

Hello Michael,

One more question regarding OPA858.

Is it a complete CMOS device (all transistors inside are MOS), or it is a BiCMOS part (most of device is bipolar with MOS/CMOS/JFET input transistors)?

I need it for thinking about future use of it.

Thank you

With a few exceptions in high speed op amps (OPA3xx series) these CMOS or JFET input devices are complementary bipolar past the input stage - that gives a much higher full power BW/quiescent current ratios.