# What is the purpose of using diode between the opamp input lines

• What is the purpose of the series diode(D2 & D1, PN: BAV199) between the current sensing opamp circuit?
• What is capacitor C3 used for?

• Input protection? – Hearth Jun 16 at 4:49
• Yeah, it's probably a clamp. I admit it does look pretty weird in this circuit though. – DKNguyen Jun 16 at 5:03
• It’s hard to see how it’s input protection, given that the inverting input will tend to follow the non-inverting input in normal operation. – Frog Jun 16 at 7:14
• @Frog perhaps some people also consider abnormal operation and try to protect the devices from it. – Arsenal Jun 16 at 13:11
• @Arsenal for sure but this setup would only ensure that both inputs get zapped at the same time, which I suppose avoids the ignominy of having to replace an op amp because only one of the inputs is blown – Frog Jun 17 at 5:38

The connection of the diodes looks weird to me. I'd say "they are clamping diodes" but the connection should be anti-parallel across the inputs:

simulate this circuit – Schematic created using CircuitLab

also put some light on why Capacitor C3 is used.

Without C3, the circuit will be a non-inverting amplifier. With C3, the circuit will be an active low-pass filter with non-inverting amplification. The cut-off frequency is

$$\mathrm{ f_C=\frac{1}{2\pi \ 75k\Omega \ 1nF}\approx 2.1kHz }$$

and the pass-band gain is

$$\mathrm{ A_v=1+\frac{75k\Omega}{2.87k\Omega} = 27.1 }$$

• Diodes are not in anti parallel, they are series diodes connected between inputs. Is there anything than clamping? – Rohan Kharvi Jun 16 at 13:11

It produces that instead to clamping it to ground it clamps it to the inverted voltage such that the opamp output would be the maximum discrete possible value which can be interpreted by an mcu or a comparator to trigger a signal to the input line control itself...

• Welcome to EE.SE, nick. Your answer seems a bit confused. The op-amp is powered by a 5 V supply so it's not likely that it can cause any problem to an MCU. It's more likely that it would be to prevent the op-amp latching up. – Transistor Jun 16 at 21:21
• oops. my mistake. it's a 1.2 volt clamp forming a self regulating high pass. it clamps the max input below popular line input 1.7 volts at 1.2 aprox and operates from there in the avalanche region,also making use of the gain resistor as a fuse and current clamp. brilliant, – nick Jun 16 at 21:29