AD7763: Power dissipation

Question We are about to decide an A/D converter for one of our applications. One of the strongest alternatives is AD7763, but I have a question about this component: In the data-sheet junction-ambient thermal impedance is shown as 92.7 C/W. Also the part has industrial operating temperature range of -40/+85. The power dissipation of the device is neraly 1 watt. Then 85C (ambient temperature) + 92.7C/W x 1W (junction to ambient temperature at 1 watt) ~=177C which is higher than the maximum allowable junction temperature 150C. How can we solve this problem? (Is the effect of thermal paddle included in these numbers? Or otherwise do we need external cooling techniques?) Answer θja for the AD7763 is indeed almost 100ºC/W but this is the value with the paddle unconnected. Since the device should never be used without its paddle being soldered, or otherwise thermally connected, to a PCB plane or some other heat sink this high value of θja is unimportant. θjc, however, is only 5.1ºC/W so if you have a reasonable value of θ (paddle-ambient) you will easily be able to use the device at 85ºC.

AD7763: Multiplexing the input using external mux

Question I wish to multiplex 8 channels to 16 channels and require advise on the best time to change over the analog switch during the AD7763 cycle as it is unclear on the current datasheet when the AD7763 starts processing the analog. ie at the fall of Drdy or fall of FSO. Answer The AD7763 is a sigma delta converter and the input is sampling continuously. In order to multiplex the input, you need to reset the digital filter, so that results from the previous channel do not interfere with results from the new channel. Given that the filter delay is many 10s or hundreds of us, a change of channel between each conversion and the next one dramatically reduces the sample rate that can be obtained; the overhead each time the channel is changed is very significant. The AD7763 starts processing the signal as soon as the SYNC\ signal is deasserted. The first valid data is indicated by the DVALID status bit i.e. after the digital filter has settled. Once this is the case you can read one or mode valid results and then change the channel on the mux, toggle the SYNC\ pin and start again. DRDY is asserted at the output word rate, but the first output words after a channel change are not valid. FSO is just a frame sync for the output data. It doesn't indicate anything about the conversion status.

As in the attached figure, the voltage at terminal A and B of AD7763 in our application can be -2.5V. Is this OK?

Q As in the attached figure, the voltage at terminal A and B of AD7763 in our application can be -2.5V. Is this OK? A The input can be at the minimum of -2.5V at the terminals A and B. This is because – the output of the differential amplifier VoutA+ and VoutA- are always biased to 2.048V. –this means that this DC bias will feed back through the components RFB to the in input pins of the amplifier. This will DC bias the analog input to the amplifier to a value close to ground. Attachments:

Downloading evaluation software for AD7763 Evaluation Board from CD

When downloading the evaluation software for AD7763 from the CD you may come across the following error showing when the install is ~25% through the completion bar. This error is a remnant of the transfer of the installer from CD ROM to your PC. Running the same installer downloaded from a USB stick, or downloaded from the AD7763 evaluation board page will not show up. this proves that the error is related to the CD ROM transfer. Advice here is to click on "Ignore". Continue the install. On completion of the install there is no effect on the operation of the evaluation kit and the software runs correctly.

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AD7763 Production
The AD7763 high performance, 24-bit, Î£-Î” analog-to-digital converter (ADC) combines wide input bandwidth and high speed with the benefits of Î£-Î” conversion...
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