LM258 circuit help

网络游侠2018

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Could you please analyze the principle of this analog quantity acquisition?

maychang

The values of the resistors in the figure may be wrong. In addition, after the first stage of amplification (gain is only 1.2 times), there is a follower stage, and the output is connected in series with a 1 kilo-ohm resistor. I don't understand what the author means.

网络游侠2018

maychang posted on 2018-12-14 18:30 The values of the resistors in the figure may be wrong. In addition, after the first stage of amplification (gain is only 1.2 times), there is a follower, and the output is connected in series with a 1 kilo-ohm resistor. I don't know...

It was left by my colleague, but I didn't leave the design.

maychang

Network Ranger 2018 Posted on 2018-12-14 22:13 It was originally left by a colleague, but no design was left

In this case, there is no need to pay attention to this circuit. If you need to use an analog quantity acquisition circuit, redesign it yourself according to the actual requirements.

chunyang

The front-end circuit of the op amp for analog quantity acquisition is usually a buffer amplifier. The original poster can design it according to the actual needs. In addition, the op amp circuit must be drawn according to the rules, not as shown in the original poster's picture.

PowerAnts

maychang posted on 2018-12-14 18:30 The values of the resistors in the figure may be wrong. In addition, after the first stage of amplification (gain is only 1.2 times), there is a follower, and the output is connected in series with a 1 kilo-ohm resistor. I don’t know...

If the leftmost resistor is really 40 ohms, it means that the signal carrying capacity is not bad. Although the load behind the emitter follower is not light, it is definitely much lighter than 40 ohms. The op amp is wasted in vain. The former colleague of the original poster must have a grudge against the boss.

PowerAnts

Unless a voltage gain of 1.2 is required

jomerwj

I feel that the follower should be placed in the front, and the amplifier should be placed in the back stage. And the OP did not amplify it. Are the parameters wrong?

w522930954

The resistance parameters should be filled randomly and should be based on the actual situation.

xiaxingxing

PowerAnts posted on 2018-12-16 23:02 If the leftmost resistor is really 40 ohms, it means that the signal carrying capacity is not bad. Although the load behind the emitter follower is not light, it is definitely much lighter than 40 ohms. The op amp...

Master, "Although the load behind the emitter follower is not light, it is definitely much lighter than 40 ohms. The op amp is wasted." I don't understand this sentence. Can you elaborate on it? Thank you!

maychang

xiaxingxing posted on 2019-1-3 19:27 Master, "Although the load behind the emitter follower is not light, it is definitely much lighter than 40 ohms. The op amp is wasted." I don't understand this sentence, can you elaborate? Thank you...

The impedance from OUT1 (pin 1) to ground is 40 ohms (the two input terminals of the op amp are "virtual short"), and even considering the impedance of pin 3 to ground, it is not that big, at least much smaller than the 1 kiloohm resistor in series with OUT2 (pin 7). So the emitter follower (referring to the voltage follower composed of the op amp at pins 5, 6, and 7) is useless, wasting half of the chip.

xiaxingxing

maychang Published on 2019-1-3 20:26 The impedance from OUT1 (pin 1) to ground is 40 ohms (the two input terminals of the op amp are "virtual short"). Even considering the impedance from pin 3 to ground, it is not that big, at least it is smaller than OUT2 (pin 7)...

I understand, thank you! So, for the last stage of op amp, we should not connect too large a resistor in series (when we need strong driving capability). Can we say that?

maychang

xiaxingxing posted on 2019-1-4 23:17 I understand, thank you! So, for the last stage of op amp, we should not connect too large a resistor in series (when we need strong driving capability), we can...

The requirement of "strong driving capability" actually requires a small output impedance. The first op amp (pin 1/2/3) of this circuit applies a very strong negative feedback, and the output impedance is already very small (less than 1 ohm). The second op amp (pin 5/6/7) is connected as a follower with a gain of 1, but a 1 kilo-ohm resistor is connected in series at the output end, which increases the output impedance to a very large value. The second op amp can be completely ignored. It is better than using it.

cuih168

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