i need some advice

[taliesin]

i'm having trouble taking opamp measurements .when i probe the opamp it changes the audio output of the circuit i'm working on.any ideas ?

[WM6]

i'm having trouble taking opamp measurements .when i probe the opamp it changes the audio output of the circuit i'm working on.any ideas ?

Explain exactly how do you test your opamp (scheme, points of connection, used measurement devices etc.).

[taliesin]

thanks

the present board i'm working on is a viking 7mx2.there are 5 lm358's.i have two oscopes..scopex 4d10 and a velleman hps40 .probes are 60mhz type X100.the inputs of all the inputs and outputs of the amps get distorted by the probes.no problems measuring 555's ,transistor oscillators ?

[WM6]

the present board i'm working on is a viking 7mx2.there are 5 lm358's.i have two oscopes..scopex 4d10 and a velleman hps40 .probes are 60mhz type X100.the inputs of all the inputs and outputs of the amps get distorted by the probes.no problems measuring 555's ,transistor oscillators ?

Probably your Probe work as main frequency antenna because of scope bad grounding + shielding and opamp get and amplify main frequency (50 or 60Hz). If you have opamp feedback this can happen by testing opamp output too.

You need to use proper test point too, try not directly to opamp input but more toward signal source (before passive element connected to opamp input and after feedback line).

Put here diagram of your measurement.

[taliesin]

ok thanks will do

what i'm doing is making coils for it.as i bought the detector without coils.i don't know any info about the original coil specs so i'm trying a few different combinations.
ty

[Skippy]

Try using another opamp ( fet/mosfet input) as a unity-gain buffer, then look at the output of that on your 'scope.

[taliesin]

thanks

will do skip .oh do i use the same power from the board?

[Skippy]

Using the existing supplies makes sense, but make sure your buffer amp can handle it.
You can also give the buffer some gain if you're looking at small signal levels, eg. 18K and 2K0 (or 27K & 3K0) will give you X10 with the standard non-inverting opamp configuration.
If you do probe directly, try using the X10 probe setting, the resistive and capacitive loading is less. Ensure you have a good 0V connection to the board.
If you have high frequency noise pickup, a quick solution is to put a series resistance in line with the probe tip. Example: 22K on a X1 probe will roll-off above 100kHz, without noticeably attenuating the signal.

[taliesin]

ta very much

[taliesin]

yes lot better with the buffer

thanks for advice