Analog Circuits - How much current is required on reference voltage pin of ADC

Indian 2:12 PM

For a given ADC, we know that there are two voltages to be provided. While one is supply voltage to run the ADC, the other one is the reference voltage. The following are the various scenarios that can be considered for the reference voltage (VREF):

VREF can be internal to ADC
VREF is external to ADC

If we consider, external VREF scenario, it can be same as supply voltage voltage of ADC or it can be within a given range given in the datasheet

For example, we will consider MCP3204 which is a 12-bit SAR ADC from Microchip.

If we see the functional block diagram from the datasheet of MCP3204, it requires external VREF to be provided.

The VREF range for the ADC is as below from the datasheet.

We can see that VREF can be as low as 0.25V and as high as the maximum supply voltage of the ADC. From this, one thing is clear, VREF cannot exceed supply voltage of the ADC and Analog input to the ADC cannot exceed the VREF voltage.

Now, that we have two supplies given to ADC - one if VDD and one is VREF. There will be a current draw on both these pins. As we are talking about SAR ADC here, in SAR ADC architecture, VREF switches into a capacitor bank which charge and discharge bases on the conversion requirement. So, one thing is sure that there is continuous current drain on the VREF pin. If we look at the datasheet, it also mentions that current but it is always a average current. There could be more current drawn than this during the conversion time and the designer engineers have to consider this. One thing is sure, the current will be in the range of few milli-amps.

Here is the snapshot of the current requirement for MCP3204 for the VREF pin:

We can read from the above specification that even when chip is pushed to shutdown mode using CS, there is significant current. This where quiescent current becomes a critical specification if you are using ADC in battery powered applications.

The below snapshot shows the operating current of the ADC. This is the current drawn from the VDD, which is the main power source to the ADC.