ICL 7107 digital voltmeter helps needed

William loh

I'm building ICL 7107 digital voltmeter with 200mV full scale as Figure 12 (page 11) on http://www.intersil.com/data/fn/fn3082.pdf.
Here is the problem I'm facing:
it shows random numbers, sometimes showing 1666, and it has no effects even when I try to calibrate by adjusting the potentiometer and shorting the inputs pins the values remain the same. Even the whole potentiometer is removed the value still stay there!
I have read through the datasheet and couldn't find any defects of the components that might have caused this.
I have tried to replace the 0.1 microfarad capacitor with 1 microfarad but the problem remains the same.
So, I was wondering if you have any idea? and Thanks in advance, that means a lot to me.
William

vk6kro

Have you got pins 30 and 32 joined together, and connected to ground? This is shown as a dotted line in the diagram, but it should add to the stability of the device.

You should then be able to join the input (pin 31) to ground and get a zero reading.

Have you got the center ground of the +/- 5 Volt supply grounded to the same ground as the ICL 7107 ground?

William loh

Hi,
Thanks for your advice indeed, and no, I have not got pins 30 and 32 joined together but I have joined the pin 30 to the same ground as the IC.
The problem was solved, it is because of the 'leg', I don't know what it is called, its function is that we can solder the legs and put whatever components in the legs, just like IC socket, it help us to change components easily without having to resolder and etc.
The legs though tested short by multimeter but do not have good contact with capacitor, I remove all the legs for capacitor and directly solder the capacitor in the board, the problem is then solved.
By the way, why should we connect the same ground as the IC? I just wonder as aren't all the grounds of the equipment are the same?

William

vk6kro

You might mean the IC socket. Like this:



Very short wires going to ground are important for many circuits and even the extra length of conductor in a socket can make the circuit unstable.

The actual ground is not that important, but the IC connections that should go to ground should be joined to each other as close to the IC as possible. You can do this with pieces of wire if the printed circuit board does not already do it.

Glad you got it going.