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AD7682 SPI Control

Question asked by GlennP on Mar 13, 2013
Latest reply on Mar 16, 2013 by KarenNE

I'm designing SPI interfaces to control multiple AD7682 devices on a board, each with different analog input configurations and reference connections.  The intent is to hide the complexity of the SPI interface from software by continuously accessing the A/D and storing the conversion results in RAM for software to access.  I have multiple questions that I can't find an answer to in the datasheet. 


In one case there are multiple analog muxes feeding an AD7682 that select from about 20 different voltages that need to be monitored.  The current circuit works using the sequencer but I want to speed up the sampling process so that only those inputs that are valid are sampled (the mux control is common to all analog muxes, but not all analog mux inputs are being driven).  I'm assuming sampling of inputs in a specific order can be performed using bits 9:7 in the configuration register (and turning off the sequencer in bits 2:1)? 


Other than using the sequencer, how can the temperature sensor data be accessed?  I'm also confused by bits 12:10 in the configuration register table (Table 10) because there is the "Unipolar, INx referenced to GND" that I will need to use for the external inputs, while still wanting to use the Temperature sensor, which appears to have its own "Temperature sensor" setting?


What does the "Update configuration during sequence." entry for bits 2:1 of the configuration register do?  It implies that you first select one of the sequencer settings and then this setting will be retained while changing other configuration register fields (in other words, if using the sequencer the sequence won't be disturbed when this entry is used), but there are no details in the datasheet regarding this. 


Does readback start on the SPI transfer that enables the readback capability, or is there a delay? 


If an AD7682 REF input is being driven externally but REFIN just has external caps connected, can the temperature sensor still be used by setting bits 5:2 of the config register to 0x2 (External reference, temperature enabled)?  Does this turn off the internal buffer?  In this case I'm assuming the AD7682 output would reflect that the temperature sensor voltage is compared to the externally supplied reference? 


Finally, just to make sure I understand the spec correctly, I should expect about a 1 mV change in the temperature sensor per degree C?  So if I'm using a 4V reference I would expect to see the conversion value change about 16 steps per degree C?