TIC12400-Q1: ADC for real-time input monitoring

Hi Piotr,

Yes the ADC can be used to monitor an external voltage not associated with the resistive voltage drop across a switch.  However, the device does not function differently and it will still poll each input pin that is enabled in a sequential loop.  There is an internal MUX that switches the inputs to the ADC for measurement one at a time.  The minimum polling cycle is 2ms and therefore each enabled input pin can be measured once every 2ms even if there is only one input channel enabled.  If you need to use the ADC to sample the voltage on a particular input faster than once every 2ms, then this would be a limitation.  Otherwise, you could use the ADC to measure 24 external voltages once every 2mS.

You will need to set the wetting current level to 0mA which will turn off both the current source and current sink circuits and allow the external voltage to pass through to the ADC input.  There are a couple of considerations here that I need to point out. 

First, the analog MUX that is used to pass the input pin voltage to the ADC or comparator, requires about 2.5V of headroom voltage to avoid saturation.  If the Vs supply voltage is not greater than the voltage being sampled by the ADC by more than 2.5V, the voltage measured by the ADC will be less than the true voltage at the input pin because it was compressed by the saturated MUX.  The ADC input voltage range is 6V and the datasheet has many typ curve figures showing the ADC Min and Max Codes with a specified supply voltage between 4.5V and 35V.

As an example, this is the typ curve for 1mA across a resistive load.  There are no graphs for 0mA wetting current and an external voltage, but the 1mA across the resistor forms the input voltage being sampled.  The black Min code represents the code that will be returned when the MUX has saturated because of a low supply voltage.  The red Max code represents the code that will be returned at a higher supply voltage where the MUX is not saturated. Therefore you will need to have a supply voltage large enough to prevent a saturated MUX so that your ADC will return an accurate code. This might be considered a limitation depending on the application.

The ADC input also has a resistor voltage divider to level shift large voltage levels down to a safe level that won't damage the ADC.  The device accounts for this divided down voltage to return an accurate code, but there is a resistive load that will be seen by the source that will draw some current which may drop the voltage at the input pin being measured depending on the drive capability of the source. The resistors in the divider are each the same value and the datasheet gives the total series resistance of the two resistors between the Input pin and GND which is typical 234kΩ, or each resistor is typically 117kΩ.

For example, if you had an external current sense resistor that you wanted to use the ADC to measure the voltage drop across it, then this internal resistance will effectively be in parallel with that external current sense resistor and could draw some additional current from the source and alter the measurement.  Depending on your application this may be a limitation, or just something that will need to be accounted for in the result.

I will also note, that the device already has the capability to measure the Vs supply voltage with the ADC and this bypasses the internal MUX.  But it is another example of how the ADC can be used to measure an external voltage.

I hope this answers all your questions.

Regards,

Jonathan