MC34708 ADC input specifications anyone ?

[pierreschirrer]

Hello,

We designed our own i.MX53 board using the MC34708 companion chip. Since this nice IC comes with an integrated ADC, we figured that we are going to use one of the channels to measure the external temperature around the CPU core. We therefore wired of thermistor to ADCVIN11.

I am trying to get hold of the ADC input specifications to try to derive the conversion formula to get the temperature from the ADC reading. The datasheet is very terse in giving information on the ADC input circuitry.

Does anyone have more information ?

I am looking at input impedance values, or current drawn by the input etc.

This is about all the data sheet gives :

Why is it giving a source impedance and not a load impedance ?

A more acurate value would be needed in order to derive the conversion value etc...

Thanks for your help,

Regards,

[AnandBalakrishn · Accepted solution]

Hi Pierre,

The input is quite high impedance. I am trying to find out the exact value but for practical purposes, it won't affect the readings as its just a 10-bit ADC. I will find out the values and update.

The influence of the source impedance explained in a TI app note: http://www.ti.com/lit/an/spna061/spna061.pdf

Regards,

Anand