ADC ST72C334N4

marco32 · ‎2003-07-23

Posted on July 24, 2003 at 08:09

ADC ST72C334N4

marco32 · ‎2003-07-11

Posted on July 11, 2003 at 11:22

I,ve many times tried to initialise an ADC input, just following the datasheet.

IO intialise to floating input and select the ADC channel.

The Voltage supplied to the ADC input, drops at least 1V?

It looks like an low impedance from the IO?

Anybody an suggestion?

[ This message was edited by: Moppie on 14-07-2003 15:33 ]

itsmejatinus · ‎2003-07-13

Posted on July 14, 2003 at 02:48

Is the voltage permanantly drops by 1v when applied or is it for a moment at the conversion time. Because at the conversion time there may be a spike seen on CRO.

[ This message was edited by: Ranjeet on 14-07-2003 06:19 ]

marco32 · ‎2003-07-13

Posted on July 14, 2003 at 04:45

The voltage is permantly drops by 1Volt.

Even when i don't select the ADC channel, only floating input.

I've been searching in the datasheet for the floating input impedance, but i can't find.

This is the first time i used the ST72C334.

The ADC in the ST72C254 is working well.

simonharrison9 · ‎2003-07-13

Posted on July 14, 2003 at 06:53

Hi,

I had a similar problem with one of my customers.

He was using series resistance of 100k.

If you are doing the same, either reduce it a lot

The other thing maybe a bad solder joint.

What is supplying the value - resistor divider / potentiometer etc?

regards,

Simon

Anglia

marco32 · ‎2003-07-13

Posted on July 14, 2003 at 07:45

I had used a serie resistor of 10k.

I reduced it to 1k and it works well.

Thanks.

[ This message was edited by: Moppie on 14-07-2003 11:30 ]

sisto · ‎2003-07-22

Posted on July 22, 2003 at 13:07

Hi there, You must buferize Your analog signal with a voltage follower or reduce, if you have one, the external series resistor of your eventual RC noise filter. The best choise is 10K and 0.1uF.

This appens because the ADC input impedance is not a high impedance, but the mean value is about 600-800K

simonharrison9 · ‎2003-07-22

Posted on July 23, 2003 at 08:57

Hi,

With the problem my customer had, the impedance of the ADC input calculated to over 6Mohm.

Simon

sisto · ‎2003-07-23

Posted on July 23, 2003 at 16:52

may be Simon, may be that's what Your customer said, about the problem, it the true, but the reality is that if the value of the series resistor is low (1K

Sixtus

simonharrison9 · ‎2003-07-23

Posted on July 24, 2003 at 08:09

It's basic mathematics...

you have a 5.25v supply rail, 100k series resistance that is showing a voltage drop of 850mv.

The current can be calculated, the impedance calculated from that.

The ADC readings before and after prove the fact

Simon