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Anyone can correct this circuit for me?

http://www.electronics-lab.com/projects/test/007/index.html For the input for pin 30 and 31 of the IC, the voltage divider got some ploblem. As i know, the current that flow into pin 30 & 31 should be stable, but this curcuit show that unstable current flow into pin 30 & 31, i think the resistors are wrong. why the pin 30 & 31 are short circuit? if pin 30 short to pin 31, how to measured the input?

Public Comments

1. Not sure I follow you - are you saying you've built the circuit and you're getting unstable reading with a stable input? The resistors look right to me. Maybe you could put a cap across the pot, start with maybe 0.1uF, check response on all ranges, if you'd like more filter, go to 1uF.
2. looks good to me... this circuit uses dual supply.. +5V and -5V.. the -5V is different from the GND...
3. As shown the circuit is wrong. Just to be self consistent the 1 K resistor should be 10K (I can tell because the ranges would be consistent with the resistors if the 1K was changed to 10K -- note this circuit has shown up on this site before). The resistors act as a voltage divider. The current into the pins should be too close to zero to measure so it will look unstable on a meter. The input to a voltmeter should have a very high resistance. Using the resistor divider in the circuit between pins 30 and 31 is a bad idea because this drastically lowers the input resistance of the circuit which would result in the meter loading the circuit it is measuring. The correct way to change the voltage range for this chip is to make a change at the voltage reference pins (pins 35 and 36). See figure 15 of the datasheet at the following link: http://www.intersil.com/data/fn/fn3082.pdf By the way the way the way the circuit was drawn in the link you provided the pins 30 and 31 are not exactly short circuited. The circuit designer intended the 2K resistor to be adjusted to about 1 K so that on the 2 V range the resistors would form a divide by 10 voltage divider (this is why I suggest there is a error and a 10K should be used not a 1k). In the 20 volt range the 100k forms a voltage divider with 1k for a divide by 100. Full scale occurs when 200 mV is between 30 and 31. I suggest you also look at the AN023 given below in references. To sum up if you change the 1k to 10k the circuit should work for measuring directly from a power supply or regulator. In general the voltage divider is going to load a circuit if you use this as a typical voltmeter in which case this circuit is not going to work. If you look at the datasheet and at the application note you will find typical circuits (with a fixed range).