In-depth analysis of the role of op amps

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In-depth analysis of the role of op amps

Posted Date: 2024-01-19

In-depth analysis of issues such as 4-20mA op amp selection and the impact of A/D reference voltage on measurement accuracy.

Among them, in order to provide anti-interference capability, the signal input to the op amp needs to be changed to a differential input instead of a single-ended input.

The role of op amp

Many netizens mentioned that since the op amp is only used as a voltage follower and has no amplification effect, why not omit the op amp and directly input the voltage to the A/D port of the processor.

Detection circuit without voltage follower

Detection circuit using voltage follower for signal isolation

In my opinion, in addition to not amplifying the signal, the op amp plays the following roles:

01Isolation and impedance transformation

The op amp has the characteristics of high input impedance and low output impedance.

Taking advantage of this feature, the impedance of the input and output will not affect the output and input signals.

Looking in from the 4-20mA input port, the equivalent circuit of the input impedance is as follows:

Equivalent circuit of input impedance

The input impedance is:

Among them, RIN is the input impedance of the load, and Ri is the input impedance of the 4-20mA signal;

For circuits that directly input the A/D port, RIN is the input impedance of the A/D port;

For the voltage follower circuit, RIN is the input impedance of the op amp;

In recent articles, I have always emphasized that errors that do not change with device, temperature, and time can be eliminated through calibration.

Therefore, only the error caused by changes in RIN due to changes in RIN needs to be considered.

According to the equivalent model of the op amp in the figure below, the common mode input impedance of the non-inverting terminal can be calculated as.

Op amp equivalent model

At node 1, using the node current method, the sum of all circuits flowing into the node is 0, and the following formula is established:

Simplifying by moving terms we get:

The current i is obtained as:

The input impedance of the non-inverting terminal is:

For the voltage follower, R1=∞, R2=0 we get:

Among them, rd is the open-loop input resistance, r0 is the open-loop output resistance, and α is the open-loop voltage amplification factor;

A typical rd is a megaohm level or larger, while r0 is a hundred ohm level.

From the specifications, we know:

TLV4333 specification sheet

The relationship between the open-loop amplification factor α and the open-loop gain AOL is:

the typical value is 125892.5412, and the maximum value is 3162277.66.

Therefore, the input impedance of the non-inverting terminal of the op amp TLV4333 can be considered infinite.

From the STM32F103 specification book, the I/O input current can be found as follows:

Input leakage current

It can be deduced from the leakage current that the input resistance is MΩ level;

Therefore, the input resistance of the op amp is much larger than the input resistance of the processor IO port;

Therefore, the input resistance of the processor I/O port has a much greater impact on the current sampling resistor than the input resistance of the op amp.

It should also be noted that the leakage resistance on the PCB is also a non-negligible factor that will affect the accuracy;

Especially in a humid environment and dust accumulates on the PCB, the leakage resistance may be as low as hundreds of K.

It is much smaller than the input resistance of the op amp and becomes a key factor affecting the detection accuracy.

This effect can be effectively reduced by increasing the gap between PCB traces and spraying conformal paint on the PCB.

02Filtering and anti-interference

By connecting the filter capacitor in parallel at the op amp end, an active low-pass filter is formed, which can effectively filter out high-frequency interference;

In addition, in the recent article, because there is a signal isolator in the signal front stage, all operational amplifiers use simple single-ended signal input;

Its disadvantage is that it cannot effectively filter common-mode signal interference from 4-20mA signals;

Further modify the circuit as follows:

Therefore, the single-ended input is changed to a differential input. It is important to note that the following circuit is only for illustration and cannot be actually used in products because it does not meet the requirements for isolation and common-mode input voltage.

In the next article, we will analyze the parameter values ​​and the principle of anti-interference in detail.

Differential input circuit

Review Editor: Tang Zihong

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