DAC60504: Query regarding VREF and VDD supply

Hello,

We have some more observations as below,

We prepared a same circuit on a separate general purpose board and we provided supply to VDD and VREF pins using a separate power supplies, it is observed that supply connected to VDD do not consume much current but the supply connected to VREF pin consumes very high current.

When the current limit to the supply provided to VREF pin was set to 10mA, it was dropping the voltage from 5V to 2.5V with the supply going into constant current (CC) mode. That is the VREF pin current requirement is exceeding the set current limit.

When we increase the current limit, it is observed that the VREF pin is consuming more than 50mA current. So such a high current consumption on VREF pin is expected? or the DAC is not changing configuration from internal reference to external reference?

Hi Nitesh, 

The DAC internal reference is enabled by default, so this behavior is expected. You need to update the DAC register to disable the internal reference on startup if you plan to use an external reference. I would recommend leaving the REF current limit at 10mA or adding a series current limiting resitor to limit the current in the time between power on and you disabling the internal reference. 

Best,

Katlynne Jones

Hi katlynne,

Ideally, shouldn’t the external reference mode be the default for a DAC?

As a user, if I am applying an external supply on Vref pin then I would consider that default setting of DAC would be on External mode rather Internal mode. Also, until I update the DAC register the internal reference pin i.e. 2.5V and external reference source (5V in my case) would get short and as a result would sink large current, which is visible in our testing with supply connected on Vref pin is consuming 56mA. 

Hi Nitesh, 

I believe the designers chose the default setting to be this way so the device could power on to a default voltage without the user needing to send any commands or providing external reference. This device has a pretty good internal reference, so they prioritized that function rather than assuming most people would use an external reference. Though I get where you're coming from that it causes some trouble if you do choose an external reference.

Again, in your case I would recommend leaving the REF current limit at 10mA or adding a series current limiting resistor to limit the current in the time between power on and you disabling the internal reference.

Best,

Katlynne Jones

Hi Katlynne,

Thanks for the response. Can you please check if there is a direct replacement for 'DAC60504BRTET' with external reference, so we can probably use them in future.

Regards,

Nitesh

Hi Katlynne,

Currently we are using  'DAC60504BRTET' in internal reference mode,

We have set Gain Register as 0x010F ie Vref /2 and gain * 2

 To set voltage output to 2V on DAC Channel 0, we are setting DAC count as 3276.

 Data sent on SPI is as follows:

TX Data[0]: 0x08 

TX Data[1]:0x0c  

TX Data[1]: 0xcc

Data read for DAC0 is as follows

RX_data[1] : 0x0c  

RX_data[2] : 0xc0

which is equivalent to DAC_count 3264

 When measured on DAC0 Pin, voltage we are seeing is 0.124 on multimeter.

Hi Nitesh, 

There is not a direct (pin to pin) replacement with the internal reference disabled by default. 

Had you previously confirmed you have been able to write to the DAC? Had you been able to disable the internal reference before? If not, can you ensure you are using the right SPI mode? The code to set the GAIN and REF-DIV are correct.

When you say RX data, is this just the data being shifted back out on SDO after your write? Or was this a read command of the register? 

Is this one of the same parts you had been using with the external reference? 

Best,

Katlynne Jones