enter image description here Hi We are building this circuit, and we need to protect the + input of the op-amp from over-voltage. We tried to put two diodes connected to the ground and to the supply source (Zener, regular, schotkkey) but the results we got were weird - the zener gave a steady 2.5V, schotkey and regular gave the same result, zero voltage in negative input and linear voltage at positive inputs. We did got good result (voltage clamped at +/-5V) when we disconnected Vref and R2 from the circuit.

Thank you for your help.

  • \$\begingroup\$ It sounds like you hooked up your protection diodes wrong. Can you show on a schematic how you connected them? \$\endgroup\$ – Nick Johnson May 20 '15 at 10:19
  • \$\begingroup\$ "nothing worked" and "not getting a good result"! Imagine you were at the doctors and said that. he'd say come back when you can tell me proper symptoms. \$\endgroup\$ – Andy aka May 20 '15 at 10:23
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    \$\begingroup\$ How can the voltage clamp at -5V? \$\endgroup\$ – Andy aka May 20 '15 at 10:29
  • \$\begingroup\$ Your reported results make no sense, nor does randomly substituting different types of diodes. Try again, more carefully. \$\endgroup\$ – Spehro Pefhany May 20 '15 at 12:14

The problem is obviousely in the R1 resistor. The "output resistance" of what you measure is three orders of magnitude bigger than the input resistor of your sampling circuit. What did you expect? Of course you measure the VREF. Why do you need this VREF anyway?

  • \$\begingroup\$ R2 suppose to be 82.111Kohm. We are building a scope, so we need a voltage reference to move signals to the display of the scope. \$\endgroup\$ – TzviKarp May 20 '15 at 10:36
  • \$\begingroup\$ What sort of input are you actually looking at? Maybe you should AC couple the in and output? \$\endgroup\$ – F. Bloggs May 20 '15 at 16:44
  • \$\begingroup\$ Normally you would put a +15V and -15V for your opamp and a unity gain buffer with "infinite" resistance. If you have to shift the DC level of the signal, put a capacitor in series and then apply your VREF with high resistance (depends on the capacitor and the bandwidth). \$\endgroup\$ – Gregory Kornblum May 21 '15 at 12:19

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