ADC0808 IC working correctly occasionally

Question

I am using ADC0808CCN for a project on converting analog signal to digital signal. It has an inbuilt multiplexer using which we have to provide an analog signal to one of the pin. However what happens is ;

1. Sometimes it latches to a particular value say 1000000 and remains at there no matter how many times Start of Conversion is pressed which should set the inner counter to 0.

2. Sometimes it gives slightly wrong output off by 100 or 1000 and when I press Start of conversion, it gives correct output, so its not reliable every time.

3. And most seriously , Suppose the control for multiplexer according to Datasheet are such that input pin 1 should be active, instead, it works only when the input is given to pin 5 for one particular IC and pin 3 for other.

   I have been trying to fit it in logic since 3 days however each day some new error happens. I have replaced it twice from my college lab and now the technician doesn't want to replace it anymore . What is amiss here ?

Answer 1

Posted as an answer as there is too much for a comment.

There are a number of issues here:

1 The conversion start button has not been debounced, so there will (not may) be multiple (dozens) of start conversion pulses for each button press.

2 There does not appear to be any decoupling at all.

3 The LEDs need current limiting resistors.

Devices like this need a fair amount of support circuitry to operate properly. Given the schematic you have posted, I am actually quite surprised you get any valid results.

Getting different results from different ICs is hardly a surprise here, incidentally; subtle differences device to device in a situation like this almost guarantees that.

^{simulate this circuit – Schematic created using CircuitLab}

Typical debounce circuit. Use a schmitt trigger buffer.

Answer 2

The clock frequency should be 640kHz , it is mentioned specifically in datasheet. By the way, ADC0804 has an in-built clock , you need to connect pin 3 and pin 19 by a 10k ohm resistor and put a 150pF capacitor between pin 3 and ground.