In my analogue output circuit, the following ripple voltage occurs when wired to the analogue input of a PLC with an input resistance of 69.3 kΩ.

Ripple Issue

If I put a resistor in series, this ripple disappears.

Why is that? Is it an impedance problem?

The schematic of the circuit is the next:

Output Schematic

  • \$\begingroup\$ Probably the load on the op-amp is capacitive enough to cause the amplifier to go unstable. \$\endgroup\$
    – John D
    Sep 23 '19 at 14:46
  • \$\begingroup\$ You said you put a resistor in series - where in the circuit? \$\endgroup\$
    – Reinderien
    Sep 23 '19 at 14:48
  • \$\begingroup\$ What is on the load of the U10A? What does the input and output signal look like? \$\endgroup\$
    – Voltage Spike
    Sep 23 '19 at 14:52
  • \$\begingroup\$ Resistor in serie is put between U10A:1, before the juction with U10A:2, and the load (analogue input of PLC). @Reinderien \$\endgroup\$ Sep 23 '19 at 15:02
  • \$\begingroup\$ In the load of U10A is connected an analogue PLC input. The signal is an analogue signal generated for a magnetic sensor. @VoltageSpike \$\endgroup\$ Sep 23 '19 at 15:05

The effective load on the op-amp is likely capacitive. From the LM324 datasheet:

Capacitive loads which are applied directly to the output of the amplifier reduce the loop stability margin. Values of 50 pF can be accommodated using the worst-case non-inverting unity gain connection. Large closed loop gains or resistive isolation should be used if larger load capacitance must be driven by the amplifier.

It's not too hard to pick up 50pF of stray capacitance when driving an off-board cable or interconnect.

The fact that a series resistor solves the problem is another clue that supports this.

  • \$\begingroup\$ It is also worth mentioning that putting the resistor in series with the op-amp output before the feedback connection can help with capacitive loads. However, this will also tend to increase DC offsets and decrease transient response time. \$\endgroup\$ Sep 24 '19 at 3:39

it's an oscillation proble most probably, I cannot see the sch. But a good reference for that is this book https://www.amazon.com/Design-Operational-Amplifiers-Integrated-Circuits/dp/0072320842. There is a chapter dedicated to opamp stability, if you want to know more :)


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