AD7699

BMGuardian,

Just a few quick comments that will hopefully get you started.  I'm going to assume that in this case you want to use the configuration mode to address the channel of interest for the conversion rather than use the sequencer.  However, if you do decide to use the sequencer the process doesn't really change significantly.

1) The first thing to remember is that the configuration word you write should be left justified in a sixteen bit transfer such that the MSB is a logic 1'b1 to instruct the AD7699 that you wish to update the configuration.

2) Secondly, the AD7699 has a single conversion latency (one-deep delay) between the configuration word write into the serial buffer before the buffer contents are moved into an internal register and applied to the subsequent conversion.

So in this case we are going to attempt to read from channel 1 (second channel) and assume that we have the following configuration.

• INCC:  Unipolar, with respect to COM   (3'b110)
• INX:     Channel 1    (3'b001)
• BW:     Full (1'b1)
• REF:   INTERNAL, 4.096V Buffered (3'b001)
• SEQ:  DISABLED (2'b00)
• RB:    Disabled (1'b1)

So the configuration word is  1_110_001_1_001_00_1 (0x38C9h) but we need to left justify this so the hex code becomes (0xE324h).

To update and read the results of channel 1 we then follow the process:

1. Execute a serial transfer (including toggling CNV) to write the content 0xE234h to the AD7699, being careful to observe the conversion timing for the device.
2. Initiate a conversion and read back the data at the end of conversion.  The data read back shall correspond to the channel previously programmed by the configuration word.  At the end of the conversion the configuration word you wrote in step 1 will be loaded into internal memory and the device will initiate acquisition of channel 1.
3. Initiate another conversion and read back the data at the end of conversion.  This time you should have data corresponding to channel 1.   

To verify your sequence is working you can modify the RB bit to be a zero and then request an additional word of data after the conversion result.  The AD7699 should return the configuration word on SDO if successful.  Note the MSB will be forced to 0.

I hope that helps get you back on track,

Sean

Thank you Sean, 

     But I am still struggling previously i didn't know to do this left justify of the config. But now I have changed that.

So my problem is this

I am using raspberry pi which has the GPIO 13 should be high to power up the PCB which contains the AD7699.

I have connected the AD7699 using SPI and CE0 as the chip select which is connected to CNV pin. 

So right now i have set GPIO 13 high to power up the board.

Configuration used as follows

• CFG : overwrite the contents of config (1'b1)
• INCC:  Unipolar, with respect to GND   (3'b111)
• INX:     Channel 2    (3'b010)
• BW:     Full (1'b1)
• REF:   INTERNAL, 4.096V Buffered (3'b001)
• SEQ:  DISABLED (2'b00)
• RB:    Disabled (1'b1)

CFG = 1_111_010_1_001_00_1 . Left Justified 1111 0101 0010 0100 converted to hex (0xF924)

Here's the code i used

This is the output i get 

adc, data, voltage are getting printed as per the code

Please help me with this

Sean,

Step #2 MOSI and SCLK of the first Config Transfer

Step #3 CNV/CS and SCLK for four byte transfer

Please find the attached results.

BMGuardian,

Something is not quite right with your setup as the traces are definitely not what I would have expected given your previous scope captures.  Your channel 1 and 2 data streams to be inverted versions of some arithmetic combination of the data signals we're interested in.   Can you recheck if something changed with the configuration or with your connectivity between the RaspPi and your AD7699 hardware?  

Sean

I have just removed and remade the connections. Check these new results and let me know.

Step #2 MOSI and SCLK of the first Config Transfer

Step #3 CNV/CS and SCLK for four byte transfer

BMGuardian,

So I found a mistake in my original recommendation for the configuration word.  I had said to program the device with 0xF924h which accidentally set the active channel to channel 4.  Instead try making the configuration value 0xF324 or 0xF320 (for readback).   

Also could you repeat the capture in #2 with a bit more resolution.  I would like to see how much setup/hold time we have for the data around the SCLK rising edge.  This will give us some indication if the write is working. 

I would then take a look at the MISO pin relative to SCLK on the scope after a minimum of 2 conversions to see if you readback 0x7320h (assuming readback is enabled).

Sean 

 the MISO pin relative to SCLK

Whenever I get four zeros (with readback enabled and i read 4 bytes)

Whenevr i get four 255 (with readback enabled and i read 4 bytes)

#2 with bit more resolution

BMGuardian,

I should have been more clear on the zoom I need on the last plot.  If you can really zoom in (say 100ns/div) at the rising edge of SCK when the data is transitioning that would be helpful.

Also we're going to need to start looking at other pins/voltages on your board to tease out what is going on here.  Can you please share a snapshot of your hardware schematic, at least the portions around the AD7699 so that we can get a look at how the device is connected?   Then I can offer additional measurement suggestions.

Dear Sean, 

I have attached the results at two different resolutions. I will send you the schematic as soon as possible.

This is at 100ns/div

This is at 200ns/div

I will send you the schematic as soon as I get it.

This is the schematic. We have connected the SPI pins through Raspberry pi through some isolation circuit

Also these are the results at 100ns/div and 200ns/div.

BMGuardian,

I think I misinterpreted your original problem statement as you are using an external reference and I presume desire to use the internal reference buffer.  So we're going to want to change the configuration word to modify the REF selection to use either 3'b011 or 3'b111 depending upon whether you need the temperature sensor or not.  Assuming you don't intend to use the temperature sensor then REF = 3'b11 so the new configuration word becomes 

Please note here that I've also selected channel 2 as IN0 and IN1 appear to be disconnected in your design.  I would recommend shorting those inputs to ground if they will not be used.

Please try this modified configuration word settings to see if you see any improvement in the output code accuracy and stability.

BMGuardian,

I think I misinterpreted your original problem statement as you are using an external reference and I presume desire to use the internal reference buffer.  So we're going to want to change the configuration word to modify the REF selection to use either 3'b011 or 3'b111 depending upon whether you need the temperature sensor or not.  Assuming you don't intend to use the temperature sensor then REF = 3'b11 so the new configuration word becomes 

Please note here that I've also selected channel 2 as IN0 and IN1 appear to be disconnected in your design.  I would recommend shorting those inputs to ground if they will not be used.

Please try this modified configuration word settings to see if you see any improvement in the output code accuracy and stability.

Dear Sean, 

I have changed the configuration as you suggested. But the results appear to be not changed. 

These are the results. Please check them let me know.

BMGuardian,

I would recommend attempting to slow down the data clock rate on your SPI to see if you can't get the device to respond. 

Additionally I would verify the voltages at the following nodes.

REFIN (@C19, R22)

REF (@C20)

VDD

VIO

CH2 Input.

Please also share those values with me so that I understand the static conditions you've applied

Sean

Dear Sean, 

I have changed the clock rate from 1MHz to 100kHz and these are the results now.

And the voltages at the nodes you mentioned are as fallows

REFIN (@C19, R22) - 4.1V DC

REF (@C20) - 4.1V DC

VDD - 4.9V DC

VIO - 4.9V DC

CH2 Input. - 1V DC (This is the input I'm giving in)

BMGuardian,

Can you confirm what you wrote for a configuration word?  It appears like the configuration is changing on every conversion which doesn't make sense.  

Have you tried connecting to another board similarly configured to see if that will work?

The next step will be to look at the reference and input voltages with a scope while continuously running conversions to see if there isn't something dynamically going on there that will require buffering.

Sean

I have used the same configuration which you have given, but i think I have read somewhere in the datasheet that there should not be any discontinuity in the clock after we send the configuration. But i think as per our scope the clock is getting on only while transmitting data. Is this right? This is because why i had the sending of configuration within the while loop.

I will check with other board and let you know. 

I have checked with the other device 

These are the results when I give either 1V DC or 0V.