Hello guys,
One of my customers is considering using DAC8740H instead of AD5700BCPZ-R5 because TI device is lower price.
At this moment, they have the following question.
Could you please give me your reply?
Q.
Can AD5700BCPZ-R5 be replaced with DAC8740H?
AD5700BCPZ-R5
https://www.analog.com/media/en/technical-documentation/data-sheets/AD5700_5700-1.pdf
Your reply would be much appreciated.
Best regards,
Kazuya.
Kazuya-san,
Yes the DAC8740H can be used to replace the AD5700 but it may require some small changes to the circuit. We have done this in a couple of applications and it will still require tests to pass the HART registration physical layer tests.
The first problem that I've seen is related to the HART noise sensitivity test. In those cases, the tests originally failed with some out of band interference. These failures were mostly from the low-frequency, higher-voltage level tests (125Hz @ 880mVpp, 63Hz @ 3.52mVpp and 29Hz @ 16Vpp). These tests passed with changes to the resistors and capacitors connected to the HART related pins.
For most customers this can be done with a only a few changes to the schematic. Most circuits are similar to the implementation shown in the AD5700 datasheet and also our DAC8740H schematic. If they use the external filter, the implementation is very similar.
First this is what we recommend in the TI datasheet with the external bandpass filter. It's shown on page 32, in Figure 32.
In comparison, this is what ADI recommends (AD5700 datasheet on page 14) for the input HART signal with an external filter:
And here is what ADI has for the HART output:
For the DAC8740H, the MOD_INF has a voltage divider off of an external reference of 1.0MΩ and 1.5MΩ (in comparison to the 1.2MΩ and 1.2MΩ for the AD5700). I would start with this change. This is followed by the 300pF, 150pF, and 150kΩ filter at the HART input.
For the HART output, there is a capacitor from the HART_OUTPUT pin to ground. In the datasheet schematic, it is listed as 22nF. However, in our pin list we say that it should be anywhere from 5nF to 22nF. Anything more might be unstable, and I would recommend a value at the lower end of the range (5nF). Additionally, the series capacitor is 4700pF, which would be much lower that the AD5700 value. Any evaluation could start with that, but I would recommend something higher. I think in the end, 1uF will work.
I would also note that there may be some timing issues. The AD5700 and the DAC8740H have a slightly different timing in operation.
Here is the timing for the DAC8740H:
And here is the timing for the AD5700:
For because of these minor differences, there may be issues with the HART communication and how the firmware is implemented. There may be issues because of differences in the carrier start, stop, and decay times.
Joseph Wu
