digital scales circuit AD620 op amp not working?

[moorea21]

Hi,

I'm building a machine that will utilise a load sensor from a digital weighing scale. The load sensor has a built in wheatstone bridge, using an AD620 op amp to get the signal up to a readable level. The first circuit is for this. There is a resistor network to provide a 3V reference voltage, with a decoupling capacitor.

The gain is set by the resistor on the far left.

I've checked all the input voltages, and all component values, (and my calculation for gain) and checked the wiring (breadboard) many times.

I get no output from pin 6 with respect to 0V.

Am I daft? Is this circuit no good for this chip? Or is the chip dead? I had a feeling it's my circuit, so I built the one below, with a small gain and a fixed input voltage, and the - input held to 0V. Still no output!

Attached are the circuits, and some pages from data sheet.

I have no idea why this is not working; someone out there will spot it instantly, I'm sure!

Thanks,

Richard B

 

[moorea21]

Just a note: the sense inputs should read sense- on pin 2, and sense+ on pin 3, not as they are in my diagram. Sorry! Also please excuse non standard symbols; I'm in UK, where we draw things differently, and also I haven't dealt with analogue electronics for nearly 20 years!

Richard B

 

[NSklavos]

You gain calculations for this device are incorrect.

First circuit gain = 49400/100000+1 = 1.494
Second Circuit Gain = 49400/2320+1 = 22.3