“Do you know how to calculate the input offset voltage and input offset current of the op amp? If the voltage on both input terminals of the op amp is 0V, the output terminal voltage should also be equal to 0V. But in fact, there is always some voltage at the output, which is called the offset voltage VOS.

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Do you know how to calculate the input offset voltage and input offset current of the op amp? If the voltage on both input terminals of the op amp is 0V, the output terminal voltage should also be equal to 0V. But in fact, there is always some voltage at the output, which is called the offset voltage VOS.

If the offset voltage at the output is divided by the noise gain of the circuit, the result is called the input offset voltage or the input reference offset voltage. This characteristic is usually given as VOS in the data sheet. VOS is equivalent to a voltage source connected in series with the inverting input of the op amp. A differential voltage must be applied to the two input terminals of the amplifier to produce a 0V output.

Temperature drift (Drift)

Vos changes with temperature. This phenomenon is called drift, and the magnitude of drift changes with time. The drift temperature coefficient TCVos is usually given in the data sheet, but some op amp data sheets only provide the second or largest Vos that can guarantee the safe operation of the device within the operating temperature range. The credibility of this specification is slightly worse, because TCVos may not be constant or change non-monotonously.

Vos drift or aging is usually defined in mv/month or mV/1000 hours. But this non-linear function is proportional to the square root of the time the device has been used. For example, an aging rate of 1mV/1000 hours can be converted to about 3mV/year instead of 9mV/year. The aging speed is not always given in the data sheet, even for high-precision op amps.

Input bias current (Ibs)

The input impedance of an ideal op amp is infinite, so no current will flow into the input. However, a real op amp that uses a bipolar junction transistor (BJT) in the input stage requires some operating current, which is called the bias current (IB ). There are usually two bias currents: IB+ and IB-, which flow into the two input terminals respectively. The IB value has a wide range. The bias current of a special type of op amp is as low as 60fA (approximately one electron is passed every 3us), and the bias current of some high-speed op amps can be as high as tens of mA.

Input offset current (Ios)

The manufacturing process of a monolithic op amp tends to make the two bias currents of the voltage feedback op amp equal, but it cannot guarantee that the two bias currents are equal. But there is no guarantee that the two bias currents are equal. In current feedback op amps, the asymmetric nature of the input 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 very small.

For the offset voltage, generally set a large gain and short-circuit the input. At this time, the output voltage divided by the gain is the offset voltage.

For the offset current, a large resistor is connected to the feedback loop. When S1 is closed, measure the bias current Ip at the non-inverting end, and measure the bias current In at the inverting end when S2 is closed.

The “offset” current measurement method here actually measures the “bias current” instead of the “offset current”. Of course, the “offset current” can also be known after the “bias current” at the +- terminal is measured separately. — Equal to the difference between the two.

The selection of the resistance value in the above specific example is not suitable for some situations. Generally, the bias current of the op amp is only a few nA. If a 10M resistor is used, the output voltage caused by the bias current is only a few mV, and it also includes The influence of the “offset voltage” (both large or small are possible, depending on the polarity of the offset voltage). Therefore, to measure the bias current of the order of nA, the resistance must be larger to make Ibias*R reach the v level. At this time, there is no need to deduct the influence of the offset voltage. The influence of the measurement result is only 0.5%).

How to calculate the input offset voltage and current of the op amp

Input offset voltage VIO

An ideal op amp, when the input voltage is zero, the output voltage should also be zero (no zero adjustment device). But in fact, it is difficult for its differential input stage to be completely symmetrical. Usually when the input voltage is zero, there is a certain output voltage, which is called the offset voltage VIO. At room temperature (25°C) and standard power supply voltage, when the input voltage is zero, in order to make the output voltage of the op amp zero, the offset voltage VIO is added at the input end as a compensation voltage. In fact, when the input voltage Vi=0, the output voltage Vo is converted to the negative value of the input voltage, and Vio is equivalent to a voltage source connected in series with the inverting input of the op amp. A differential voltage must be applied to the two input terminals of the amplifier to produce a 0V output.which is

Vio=-(Vo│v=0)/Avo

The size of Vio reflects the symmetry of the circuit and the potential coordination in the op amp manufacturing. The larger the Vio value, the worse the symmetry of the circuit, which is generally about ±(1~10)mV.

Vio changes with temperature. This phenomenon is called drift, and the magnitude of drift changes with time. The drift temperature coefficient TCVio is usually given in the data sheet, but some op amp data sheets only provide the second or largest Vio that can guarantee the safe operation of the device within the operating temperature range. The credibility of this specification is slightly worse, because TCVio may not be constant or change non-monotonously.

Vio drift or aging is usually defined in mV/month or mV/1,000 hours. But this non-linear function is proportional to the square root of the time the device has been used. For example, an aging rate of 1mV per 1,000 hours can be converted to approximately 3mV/year instead of 9mV/year. The aging speed is not always given in the data sheet, even for high-precision op amps.

Input offset current Ιio

In the BJT integrated circuit operational amplifier, the manufacturing process tends to make the two bias currents of the voltage feedback operational amplifier equal, but the two bias currents cannot be guaranteed to be equal. In current feedback op amps, the asymmetric nature of the input means that the two bias currents are almost always unequal.The difference between these two bias currents is the input offset current Iio. The input offset current offset current refers to the error of the bias currents of the two differential input terminals, that is, the difference between the static base currents flowing into the two input terminals of the amplifier when the output voltage is zero. Bad, that is

Iio=│Ibp-Ibn│

Due to the existence of the internal resistance of the signal source, Iio will cause an input voltage, destroy the balance of the amplifier, and make the output voltage of the amplifier non-zero. Therefore, I hope that Iio is as small as possible. It reflects the asymmetry of the effective differential pair of the input stage, generally about 1 nA~0.1 mA. The above is the calculation method of the input offset voltage and input offset current of the op amp. Bring help to designers in the learning process.

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