AMC7812B: Reference of DAC and ADC

Hello David,

David Park said:

1. Is this available? internal reference for ADC and external reference for DAC

By default the internal reference is disabled but if the ADC-REF-INT bit is set to '1' in the AMC Configuration Register 0 the internal reference will be connected internally to the ADCs. In this configuration please be aware that a compensation capacitor is required on the ADC-REF-IN / CMP pin, 4.7uF is the recommended typical value.

The DACs always use an external reference (i.e. there is no internal connection available). In cases where a user would like to apply the internal reference for the DACs, one would simply connect REF-OUT to REF-DAC.

Therefore, if desired, an application could be configured with internal reference for ADCs and external reference for DACs.

David Park said:

2. When external reference=1.25, DAC output is from 0V to 2.5V?

The DAC Gain Register contains a single-bit field for each of the 12 DACs in the AMC7812. When set to '0' (default) the DAC gain is configured as 2x VREF. When Set to '1' the DAC gain is configured as 5x VREF. For 1.25V reference then, the achievable outputs would be 0-2.5V and 0-6.25V.

David Park said:

3. Is there any advice for external reference?

Without understanding system-level goals I cannot really give complete advice. I would assume that in this case we're primarily talking about an external reference for use with the DACs - so keep in mind the requirements from the electrical characteristics table for input voltage from 1V to 2.6V and input current of 170uA typical. A string DAC reference input is an almost entirely static equivalent impedance, even during switching, so as long as the source current needs can be met there is no need to buffer as you may see with a SAR ADC.

Otherwise, the drift for the DACs are relatively low, though these are only typical values versus the max values you typically see in a reference datasheet. If drift is important this will probably be the leading decision factor for the external reference.

Meanwhile, the initial accuracy of the DAC at room temperature is about 0.2% so there should be plenty of reasonable reference sources (over half of TI's portfolio of series & shunt references is <0.2% initial accuracy, 25% exactly at 0.2%).