Does 1/4W metal film resistor have a resistance of 100M ohms?

[bancroft]

I looked around and did not see this resistance value. Is there a 100 megohm resistor for a higher power?

In the circuit below, under the amplifier, there is a 100M resistor. In the purchase of components, I do not know how to buy them. Usually, I use 1/4W metal film resistors.

 

[Bluejets]

100 meg does seem a bit high.
Where did you obtained the schematic....

 

[Alec_t]

With a value that high, any surface contamination or high humidity is likely to shunt the resistor and reduce its value considerably, by an unknown amount. I'd consider re-designing the circuit.

 

[(*steve*)]

Looks like an integrator Given the input impedance of that op amp is around 10^13 ohms, 100M isn't a lot. Sure you need good construction technique (and actually need this circuit) but there's not that much weird with it.

(There are similar circuits with much higher resistances)

A physically larger resistor will be less subject to leakage (after cleaning).

However you also need to be careful in the selection of C5 as you want something with low leakage and also keep that clean. typically polystyrene capacitors are used in these circuits but due to the relatively low 100M resistor of recommend something else (some other film capacitor should be fine) mostly because polystyrene caps can't be cleaned with alcohol, and that's probably the best solvent to get rid of finger grease.

Whilst it's not absolutely essential, if you lift the pin for the inverting input so it's sticking in the air and connect the various other components to it directly, you will eliminate many of your other leakage issues as long as you keep the IC itself clean.

 

[(*steve*)]

See the following circuit using a 1G resistor in a similar circuit http://www.vk2zay.net/article/251

 

[bancroft]

100 meg does seem a bit high.
Where did you obtained the schematic....

It is indeed a bit high, I got this schematic from a friend.

 

[bancroft]

With a value that high, any surface contamination or high humidity is likely to shunt the resistor and reduce its value considerably, by an unknown amount. I'd consider re-designing the circuit.

Thanks for your reply, I will probably abandon or redesign this circuit.

 

[bancroft]

See the following circuit using a 1G resistor in a similar circuit http://www.vk2zay.net/article/251

Ok, thanks for the suggestion, I will think about this circuit again.

 

[(*steve*)]

What do you want the circuit to do?

 

[DBingaman]

Don't see a reason for it to keep circuit operation the same. You could just as easily increase C5 by two orders of magnitude and reduce R7 by two orders of magnitude. Looks like you want an amplifier with an AC gain of over -21,000! Then add another stage with a gain of 100 to make up for it.

 

[DBingaman]

An instrumentation amplifier with two stages might be a better choice, less noisy and more reliable.