# Input voltage drop in LM358 with “an improved precision rectifier circuit.”

I found the input voltage drops around 2V drop (observed from oscilloscope) when I connect the ion mobility spectrometer (IMS) signal to the circuit "an improved precision rectifier circuit", like the attached picture.

I have already used single supply +5V from Arduino Due and dual supply +-15V, but the problem still existed.

Can someone help me to deal with this problem?

Update 2020.07.15:

I modify some parts to what you suggested. I still have some problem with my circuit. I have the positive and negative signal from the signal source. The first LM358 I use it as voltage follower. Second LM358 I use it as inverting signal circuit, and I can get the peak what I want. But I need to remove up to -4 to -5V and amplify positive signal from second LM358 output. When I use the third LM358 as below, the positive signal is amplified but the negative signal also amplified. Can I have some way to remove negative signal and let the positive signal be amplified?

Second question is I found the signal decrease before first LM358 like picture shown below when I connect the wire between Arduino and signalinput.

• What is your input signal frequency range and what is it's output impedance? – Andy aka Jul 13 '20 at 9:35
• Maybe I need to ask the instrument company first and answer this question! – Chi-Hsiang Jul 15 '20 at 14:22
• @JRE Thanks for reminding. I am a beginner on this website. I will be careful next time. – Chi-Hsiang Jul 15 '20 at 14:46
• @Andy aka input signal frequency is 150000 Hz and impedance is 1kOhm. – Chi-Hsiang Aug 5 '20 at 9:23

You cannot get gain from this circuit (R1 must equal R2) and generally you may need a negative supply voltage, depending on the op-amp, though not with single supply op-amp.

If you use the second half of the op-amp to amplify the signal, you can get it to work with small input voltages, say up to 100mV peak.

The OA1 circuit is similar to yours, but the resistors are equal and there is no need for the second diode since the output cannot go below 0V. The second amplifier is used as a non-inverting gain of 31 amplifier.

This circuit would benefit from a better op-amp (lower input bias current and rail-to-rail output). The maximum input is limited by the maximum output of OA2, around 3V or a bit more.

simulate this circuit – Schematic created using CircuitLab

• You can get gain from this circuit as far as I can see. Maybe you mean something else? – Andy aka Jul 15 '20 at 14:24
• @Andyaka you cannot have gain and have it properly function as a symmetric full-wave rectifier because there can be no gain for positive half-cycles. – Spehro Pefhany Jul 15 '20 at 14:27
• The OP's circuit is a half wave rectifier not a full wave rectifier. – Andy aka Jul 15 '20 at 14:28
• @SpehroPefhany Sorry for the late reply and I have modified the circuit but problem still existed. Thanks for answering again. – Chi-Hsiang Jul 15 '20 at 14:32
• @Andyaka Not really, it let positive half-cycles through with a gain of 1 up to 0.6V. One can argue it should have had a negative supply or it should be a gain of 1 full wave rectifier. – Spehro Pefhany Jul 15 '20 at 14:42