AD7792: Using multiple device on the serial bus

Question I am currently using the AD7792 converter on a board together with some other devices on the common SPI bus, however when designing I didn't realize that the AD7792 will pull it's DOUT pin low whenever a conversion is complete. Wont there be a collision if I happen to talk to another device on the bus?. Answer The AD7792 will pull its DOUT low as this line also acts as a data rdy line, the only way around this when using multiple devices on the one serial bus is to de-select the device using the /CS.. CS can be used to select the ADC in systems with more than one device on the serial bus.

AD7792: AVDD monitor and temperature sensor channel

Question We intend to use AD7792 in a new product and I am implementing the software that communicates with the AD7792. The only documentation I can find on your web site is the data sheet. I can not find all the information I need so I would like to know if you have any application notes that describes: -How to use AVdd monitor channel. Is calibration necessary and so on? Is ít neccessary to calibrate again when switching from the AVdd monitor channel (Gain is changed since AVdd monitor uses gain 1/6)? The AVdd monitor does not have its own calibration registers? -How to use the internal temp sensor. Necessary to calibrate? The temp sensor does not have its own calibration registers? Answer Q1. AVdd monitor channel is described on page 26 of the datasheet. It is treated the same as any other channel - only it has a fixed gain and internal reference. If you need an accurate voltage measurement then you need to carry out an internal zero scale and full scale calibration, as with any other channel. You obviously can't do a zero scale system calibration because the device is off when VDD is 0. The AVDD channel has its own calibration registers like any other channel, so you only need to recalibrate if the temperature changes significantly etc. Q2. The temperature sensor is an output from the on-chip bandgap reference so it measures chip temperature and not ambient temperature. It is not calibrated in production so to take accurate measurements you need to do your own calibration. To calibrate at room temperature it is probably easier to calibrate in software because 25degC is not near either end point of the ADC range (so you can't easily to a zero scale or full scale system calibration). To have a rough idea of the temperature, you can simply store the reading at room temperature and then observe any differences which occur later. I don't think it is worth doing any precision calibration because the basic repeatability of the measurement is not so good. A typical application would be to tell if the temperature has changed significantly, in order to carry out a recalibration of your system etc.

Long term drift of the AD7792 Reference

Question Can you provide any information on the Long term drift of the AD7792 Reference? Answer A typical long term drift figure would be 50ppm/1000hrs for the reference on the AD7792.

AD7792: Communication problems

Question I have the problem that the AD7792 doesn't talk to me over SPI. I try to read out the ID register as a start. The /CS is correctly low, the read-command is transmitted correctly and there are clock pulses for the response but no data is coming out. What could be the problem ? Answer If you don't manage to get any reaction from the AD7792, you might like to consider a few possible reasons for the breakdown in communications. Power supply to AD7792 If the power supply to the AD7792 comes from the CPU board and passes throught the cable then you will have a lot of inductance between the AD7792 and the power supply/ground connections. You need to decouple the AD7792 carefully as recommended in the datasheet. Its also a good idea to use several of the lines in the flat cable as power and several as ground. In this way you reduce the parasitic inductance. If this is the problem then you can easily see the heavily distorted signals on the SPI bus. - Crosstalk. Capacitive cross talk from one signal to the other in the flat cable can be avoided by alternating signal lines with ground lines and or power suppy lines. This avoids the SCLK coupling onto the other signals. - different common mode voltage at either end of the cable You need to make sure that the AD7792 and the micro ground references have some common connection.

AD7792: On-Chip temperature

Question We use AD7792 for our RTD measurement system. Page 4 (REV A datashee) show characteristics for "TEMPERATURE SENSOR". Could you please explain me what is this about ? Does it means that the chip has got an embedded temperature sensor ? If yes how can we use it (no registers to read the sensor). Answer The AD7792 has an on-chip temperature sensor. By setting the bits CH2 - CH0 to 110 respectively, the on-chip temp sensor is connected to the sigma delta modulator. When the bits are set as above, data from the temp sensor will be converted by the ADC. The temp sensor has good relative accuracy. However, you need to calibrate the sensor so that the actual value read back accurately reflects the actual temperature. The registers are shown on page 18 of the Revision A datasheet.

Do I need to perform calibrations for each channel and do I need to do this each time I change channel?

Q Do I need to perform calibrations for each channel and do I need to do this each time I change channel? A With the AD7792, there is no need to calibrate the offset as the ADC takes care of this. A full-scale calibration is required for each channel. So, as part of the initialization routine, each channel to be used should be selected, the gain, output data rate for the channel set and an internal FS calibration performed. Each channel has its own offset and gain registers. Therefore, each channel only needs to be calibrated as part of the initialization routine. When a channel is selected, the offset and gain registers for that channel are automatically selected. Further calibrations are only needed if the customer wishes to re-calibrate because the temperature has changed significantly. However, for many applications, additional calibrations are not required. It really depends on the error budget for the application. However, the calibration on power up is required. So, the initialization routine is Select channel 1 Calibrate channel 1 Select channel 2 Calibrate channel 2 Select channel 3 Calibrate channel 3 The conversion routine will be Select channel 1 Wait until DRDY goes low (120 ms when the 16.7 Hz output data rate is used) Read conversion Select channel 2 Wait until DRDY goes low Read conversion Etc.

AD7792: Single vs. Continuous Conversion Time

Q Do you have any data on Single vs. Continuous Conversion Time? A The conversion time depends on if you are using single conversion mode or continuous conversion mode. For continuous conversion the first conversion is available after a period of 2/Fadc, subsequent conversions are available at a time of 1/Fadc. For single conversion mode, conversion takes a time of 2/Fadc. The reason for the difference in conversion time is because AD7792 is a sigma delta converter and the digital filter has a settling time in response to abrupt changes at the input e.g. a single conversion/a channel change etc. In these cases the AD7792 discards the first unsettled result and outputs only the second result which has settled. In continuous conversion mode the ADC only discards the first result. Thereafter, all the output words from the digital filter are valid output.

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The AD7792/AD7793Â are low power, low noise, complete analog front ends for high precision measurement applications. The AD7792/AD7793 contain a low noise...
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