# Help for interpreting Opamps on a schematic

• eigenmax
In summary, the circuit has two identical op-amps, one resistor is too high, and there are some errors on the circuit board.

#### eigenmax

Hello,
I've been doing electronics for a while now, and only recently started using ICs. I have no problem with the 555 timer (I've used it in several circuits), but recently found and attempted the construction of a few circuits with the LM358 op-amp. On each there were two triangles, which I know represented the ICs. But when I saw an image of the circuit, I could only see one chip. I also noticed that none of the pins are repeated. Are both triangles one actual chip? Are the two triangles just ways to represent a single, physical chip?
Here's the circuit I was constructing.

#### Attachments

• ionsens2.gif
10.6 KB · Views: 1,945
Yes, that chip has two identical OpAmps inside.

Some chips even has four. On schematics, you have to be careful with the power pins, since sometimes they are represented by a third (fifth) triangle (one without the usual input-output pins).

jim hardy
Thank you, Rive! So that 1M, 10k resistor network is simply connected between pin 7 and 1. Thanks!

Be careful, this schematic is not a simple one. Was there any build instructions around where it come from?

jim hardy
I question the 100,000 megohm resistor. I once needed a 100 megohm resistor and had difficulty finding one.

berkeman
skeptic2 said:
I question the 100,000 megohm resistor. I once needed a 100 megohm resistor and had difficulty finding one.
Yeah, and there's a pretty serious error at pin 3 of the opamp external circuit. I'd make it a Quiz Question for the OP, but he's already said he is fairly new to electronics.

@eigenmax -- for potentiometers (the adjustable resistor connected to pin 3 of the opamp), there is usually a minimum wiper current specified in the datasheet to ensure reliable operation over the lifetime of the circuit that uses the potentiometer. If this minimum wiper current is not met, the wiper connection to the resistor inside can corrode over time, making the contact unreliable. So it is generally a design error to connect just the input of an opamp to the wiper of a pot. The opamp inputs generally do not have a high enough Input Current to meet the minimum wiper current spec. As an exercise, can you check the datasheet for a typical pot that might be used in this circuit, and compare that to the Input Current (not the Input Offset Current) spec for that opamp in its datasheet? Let us know what you find.

dlgoff and Asymptotic
skeptic2 said:
I question the 100,000 megohm resistor. I once needed a 100 megohm resistor and had difficulty finding one.
Google up "glass Victoreen resistor". Not an easy find...

I guess the source of the circuit is this: http://www.techlib.com/science/ionpage2.html
Should be a working model, but sure not a professional design...

Ps.: and also sure not for beginners...

Paul Colby, Asymptotic and berkeman
I stand corrected. I had no idea they existed.

skeptic2 said:
I stand corrected. I had no idea they existed.
I'm guessing that they can only be used inside hermetically sealed enclosures that have been evacuated or have extremely low humidity...

skeptic2 said:
I stand corrected. I had no idea they existed.
Not a common stuff. That page makes a good read. Things like this:
Make sure that no epoxy touches the center lead of the transistor (base lead). The epoxy is too conductive!
The resistance there is waaaay higher than what a common fingerprint has...

Sorry for the late reply. I'm using an old 100M resistor I removed from a broken piece of old electronics. I built the circuit, but realized my meter was probably not nearly sensitive enough (old analog 1-6V meter). I'll check the potentiometer thing now.

The current from your ion chamber is so feeble it's easy for it to get lost.
I'd keep this junction up in the air, not touching the board.

I don't recognize that partial arc symbol - perhaps a guard if on the circuit board?
or perhaps a metal shield covering those connections to prevent pickup of line frequency 'hum'.?

Measuring picoamps is difficult at best. Here's a snip from TI electrometer amplifier datasheet

here's a TI app note about working with high impedance circuits
http://www.ti.com/lit/an/snoa664/snoa664.pdf

Good Luck !

old jim

#### Attachments

• ionization_chamber.jpg
16.9 KB · Views: 1,477
15.3 KB · Views: 1,110
10.7 KB · Views: 1,211
dlgoff, berkeman and Asymptotic
jim hardy said:
The current from your ion chamber is so feeble it's easy for it to get lost.
I'd keep this junction up in the air, not touching the board.
View attachment 225441
I don't recognize that partial arc symbol - perhaps a guard if on the circuit board?
or perhaps a metal shield covering those connections to prevent pickup of line frequency 'hum'.?

Measuring picoamps is difficult at best. Here's a snip from TI electrometer amplifier datasheet
View attachment 225443

here's a TI app note about working with high impedance circuits
http://www.ti.com/lit/an/snoa664/snoa664.pdf
View attachment 225444

Good Luck !

old jim
The arc symbol is the FET casing I believe.

eigenmax said:
The arc symbol is the FET casing I believe.
Good catch !

Thanks !