How to measure the input offset current, how to use the auxiliary op amp, and what is the principle?

1. The total input offset current is due to the asymmetry of the input stage of the op amp, and the op amp input stage is usually a differential input pair, and its transistor or MOSFET cannot be completely consistent, so the bias voltage and bias current are not exactly the same.

The offset current is calculated as the difference in the input bias, which can be positive or negative, reflecting the degree of inconsistency of the input stage.

Look for a book on Analog Electronics.

Parameters like this are generally not focused on memorization, and there is no information at hand, sorry.

The input offset voltage UIO of the op amp is defined as: when the output DC voltage of the op amp is zero, the compensation voltage applied between the two inputs is called the input offset voltage. --The input offset voltage is generally in the order of mv.

The op amp using a bipolar transistor as the input stage has a UIO of about 1-10mV; The high-precision, low-drift op amp UIO is generally lower than ; The UIO of the op amp using the FET as the input stage is generally much larger than that of the bipolar input stage.

The input offset current of the op amp is defined as the difference between the bias currents at the two inputs when the output voltage of the op amp is zero, i.e., iio=iib1-iib2. --In general, the greater the bias current of the op amp, the greater the input offset current.

The input offset voltage, input offset current, and input bias current are all functions of temperature, and are also related to the supply voltage in actual use and the common-mode voltage added to the op amp input. The values given in the data sheet are usually measured at standard voltage values as well as zero common-mode input voltages.

Operating current: the maximum current that can be provided in normal operation, the offset current is: the manufacturing process of the monolithic op amp tends to make the two bias voltages of the voltage feedback op amp equal, but the two bias currents cannot be guaranteed to be equal.

In current feedback op amps, the asymmetrical nature of the inputs means that the two bias currents are almost always unequal. The difference between these two bias currents is the input offset current IOS, which is usually small.

It is recommended that you use the ICL7650, the input offset voltage of this op amp is only typical when the input offset voltage is only the time, and the input offset current is only.

To measure a weak current, the current signal is converted into a voltage signal with a high-resistance and low-noise resistor, and then amplified with an op amp. A 100K sampling resistor can convert a current of 10 Na to 10 mV to a voltage of 10 mV, and it is no problem to amplify this voltage with the ICL7650.

The OP27 or OP37 simply does not meet your requirements, and the input offset current of these two op amps can reach 75 nA

You can use the AD517, and its input offset current is only, which basically meets your requirements.

It is OK to use this op amp to form a charge amplifier.

But I'm afraid you're going to face a serious problem:

This signal is easily drowned out by the 50Hz AC interference signal of the power grid, how do you distinguish it from the 50Hz AC interference signal of the power grid?

You can use the OPA333 type (2333 is a dual op amp op amp with two 333 op amps) micropower high-precision full-width op amp is the best. It's VIO=2UV, IIB=70Pa, perfect for amplifying your signal. OP27 37 Although their offset voltage is only a few microvolts, the offset current reaches tens of na, and with their amplification, your useful signal will be overwhelmed by the additional offset voltage generated by the offset current.

In addition, the operation is cheap and easy to buy. The 7650 op amps are more expensive and are mostly in 14-pin packages (several of which are empty).

Addendum: Here are the OPA333 parameters for you to see. Technical specifications of OPA333AIDCKT.

Product model: OPA333AIDCKT

Number of channels: 1 Shutdown function: no

The operating voltage is max (v)

The working voltage is min (v)

IQ per channel (typical) (mA).

Bandwidth GBW (typical) (MHz).

Slew Rate (typical) (v us).

Input offset voltage (25) (max(mv) offset drift (typical) (UV).

Input bias current (max)(pa):200

Common-mode rejection ratio (min.))(db):106

Noise voltage (typical).

Single-supply operation: yes

Full Width: Yes

Package Temperature( )SC70-5 -40 125 Description: Zero drift, rail-to-rail single CMOS op amp.