Strange behavior of AD7730

[_Mark81_]

Hello,

this is the first time I write here. I apologize if my English is not so good, but it's not my primary language.

I experience some problems with the AD7730 and a small AVR. I read through the older threads in this forum but I wasn't able to fix the issue.

This is the schematic:

Some key points:

- I can successfully read the AD7730 registers but it seems I can't write them.

- I don't see any wave on CLK pins. IN is low and OUT high (steady)

- if I write to a register (eg. DAC or filter) I always read the initial value.

- if I start a conversion or a calibration I see the RDY pin that goes low for 250 ns and then returns high.

With these elements may you help me to find out the problem?

Thanks
Marco

 

[bobcat1]

HI

First disconnect SPI Din & Dout and short them together to test the SPI link

if its working (by sending a byte and reading it back at the same time)

It is most likely to have a timing problem connect SPI signal to 2 ch scope and test the signals for sync (CLK with DIN ,CLK with Dout,CLK with CS etc...)

All the best

Bobi

The microcontroller specialist

 

[_Mark81_]

Thanks for your answer.
Anyway, I've just found out the problem in the meanwhile I was reading your post.

Look at the schematic: I forgot to connect the GND to AGND!
That's why it didn't work!

Marco

Added after 2 hours 47 minutes:

Ok, now I can successfully read and write the registers of AD7730.
I performed the internal calibration, wrote the right settings on filter and mode registers and I'm starting to acquire data.

I built a simple bridge with four 1% 470 ohm resistors so with a +5V supply voltage I get about 3 mV on the ADC input.

Anyway, I always read 0x000000 from data register.

My oscilloscope tells me that the configuration seems ok: the RDY pin goes down at 25 Hz, according to the filter settings. And the readback of all register is coherent with my settings.

I tried to change the settings, for example CHOP/NO CHOP, output rate, unipolar/bipolar, input range, with or without internal calibration, but the reading is always 0.

Again, I removed the bridge, so the input pins are floating. I expect a lot of noise. Instead, I get 0....


How to investigate on this?
Thanks again

Marco

Added after 2 hours 21 minutes:

Some progresses...

I set the wrong value in the filter register. Now its value is 0x800010, that is 50 Hz output and CHOP mode.

With my resistor bridge I get a count of 0x7C11 (16 bit, bipolar, range 80 mV) that is the value of the differential voltage applied at the ADC (about 2,8 mV).

Now the strange thing: if I reverse my bridge, I apply the positive voltage to the negative input and viceversa so I expect a count of about 0x83EF. Instead I read again 0x7C11.

I miserunderstood the datasheet?

 

[szovhoz]

this is because they do not use Bascom worked very well for me this spi bus!