OPA320: Input Bias Current vs Temperature

Ohno-san,

Required specifications: max 10 pA @ 70°C. OPA320 is max 50pA at -40°C to 85°C , but it seems that there is no temperature drift up to 70°C in Figure 10. Input Bias Current vs Temperature in the data sheet.

The OPA320 is a CMOS input operational amplifier. The input bias current is dominated by the leakage current associated with the reverse-biased input ESD current steering diodes connected from the inputs, to the internal supply rails. Thus, Figure 10, the Input Bias Current vs Temperature curve is that of the reverse-biased diode leakage observed across temperature. It is a typical diode curve where the leakage current approximately doubles for every 10°C increase in temperature. The curve assures that the leakage will always be higher at 85°C, than it is at 70°C.

I will ask for the following.

 ・ Figure 10. Expanding vertical axis of Input Bias Current vs Temperature.(Same as OPA340 data sheet Figure 13.)

OPA320 Figure 10 has a linear vertical scale with a maximum of 1300 pA (150 C max), compared to the OPA340 Figure 13 which has a logarithmic vertical scale with a maximum of 1000 pA (125°C). I agree that the log scale provides finer resolution in the portion of the diode curve where the current begins to quickly change. Keep in mind that these are "typical" curves and the input bias current levels will be different for each device.

 ・ Input Bias Current Max. Value (Ta = -10°C to 70°C)

The Electrical Characteristics table states that the maximum input bias current over the temperature, range TA = –40°C to 85°C, is  ±50 pA. Therefore, since the TA = -10°C to 70°C temperature falls within that wider range the maximum is the same ±50 pA.

I will check and see if there is another op amp that has an Ib specification that assures a maximum of +/-10 pA, over TA = -10°C to 70°C. Are there any other critical parameters?

Regards, Thomas

Precision Amplifiers Applications Engineering

Thomas-san,

Thank you for answering.
I understand OPA 320.

However, why OPA 340 is Max 60pA @ 85°C regulation?
If it is +10pA at TA = 25°C, I think that +60pA will be exceeded at 85°C.

My customers would like to know.
I appreciate your support.

Best Regards,

Kenji

Hello Kenji,

The customer asked a valid question so I contacted the engineer that designed the OPA340. He explained that the OPA340 was developed over 20 years ago when our ability to measure picoampere current levels at probe was more limited than it is today. The probe technololgy of the time was able to accurately measure current levels of 10 picoamperes at room temperature, but fractions of a picoampere measurements were relegated to more-specialized product characterization set-ups. Thus, the Electrical Characteristics Table lists a typical of +/-0.2 pA, and Typical Curves - Figure 13. Input Bias Current vs Temperature, shows the results for a typical device having the typical bias current of 0.2 pA at 25 C.

Since the bias current approximately doubles every ten degrees celsius it is easier to measure the +/-60 pA maximum at 85 C probe test, and be certain of that value compared to much lower values measured at lower temperatures. If the input bias current measures 60 pA, the maximum at 85 C, the 25 C value will be a little less than 1 pA.

Regards, Thomas
Precision Amplifiers Applications Engineering