vcm spread with AD9697

Hi Marc,

Thanks for your interest in AD9697. 1.4V is the correct common mode voltage. Could you provide your SPI writes for the DC coupling configuration?

To export the common-mode voltage to the VREF pin need write some additional registers through SPI.

Regards,

Alvin

Thank you for your response.

What I write so for is :

Reg 0x1908 -> 0x04 : analog input optimized for DC coupling
Reg 0x18a6 -> 0x00 : Vref internal
Reg 0x18e6 -> 0x00 : Temp diode High Z
Reg 0x1910 -> 0x0d : 1.7V pp input
Reg 0x18e3 -> 0x4f : external Vcm buffer + some unknown parameter
Reg 0x1a4c -> 0x0f : Buffer current P : 300 uA (don't know what it means)
Reg 0x1a4d -> 0x0f : Buffer current N : 300 uA (don't know what it means)
Reg 0x1b03 -> 0x02 : Buffer control 3 (as in the datasheet)
Reg 0x1b08 -> 0xc1 : Buffer control 4 (as in the datasheet)
Reg 0x1b10 -> 0x00 : Buffer control 5 (as in the datasheet)
Reg 0x0561 -> 0x00 : offset binary, not inverted

Some parameters are taken from the datasheet, even if I don't understand in details what they really means as "buffer control".

Best regards

Marc

Hi Marc,

Thanks for the update.

Could you try below writes:

Reg 0x1908 -> 0x04 : analog input optimized for DC coupling
Reg 0x18a6 -> 0x00 : Vref internal
Reg 0x18e6 -> 0x00 : Temp diode High Z
Reg 0x1910 -> 0x0d : 1.7V pp input

Reg 0x18e0 -> 0x02 : 
Reg 0x18e1 -> 0x14 : 
Reg 0x18e2 -> 0x14 :
Reg 0x18e3 -> 0x40 :

Reg 0x18e3 -> 0x4f : external Vcm buffer + some unknown parameter
Reg 0x1a4c -> 0x0f : Buffer current P : 300 uA (don't know what it means)
Reg 0x1a4d -> 0x0f : Buffer current N : 300 uA (don't know what it means)
Reg 0x1b03 -> 0x02 : Buffer control 3 (as in the datasheet)
Reg 0x1b08 -> 0xc1 : Buffer control 4 (as in the datasheet)
Reg 0x1b10 -> 0x00 : Buffer control 5 (as in the datasheet)
Reg 0x0561 -> 0x00 : offset binary, not inverted

Please let me know if it works on your board.

Regards,
Alvin

Great !

I have added the setting of registers 0x18e0, 0x18e1 and 0x18e2 but kept the content of 0x18e3 to 0x4f and not 0x40 and it works much better.

Now I have 1.40V, 1.41V and 1.38V for Vcm of the 3 ADCs.
With 0x18e3 set at 0x40, I got 0.8V for Vcm which is not the expected value. I have understood that the 6 lsb of this register are setting a current in a buffer. I guess that this current should not be 0.

By the way I have not seen the description of registers 0x18e0, 0x18e1 and 0x18e2 in the datasheet. My version is Rev.0. I have not found newer versions.

The by-product of the new setting is also curing a problem I was fighting for a while : very low frequency instability of the baseline of the signal. Now, it is much more stable. I am looking for unipolar pulses, so the base line has to be set near the lower or upper rail of the ADC depending of the pulse sign.

Thank you very much for your very valuable help which make me a (big) step closer to a working board.

Best regards,

Marc

OK, the registers are described in the datasheet of AD9695. A new Rev of AD9697 datasheet could be useful. If I remember well, I have found some small typo in the Rev0 which should be corrected.

Marc

Hi Marc,

Good to know it works. Thanks for the updating.

Thanks for the update. Please refer to ad9695 datasheet for now, the datasheet of ad9697 is in the queue waiting to be edited. It's recommended to write 0x18e3[6] first, then write 0x18e3[5:0] as I attached earlier, the input common mode voltage will slightly change based on the buffer current settings.

Reg 0x18e3 -> 0x40 :

Reg 0x18e3 -> 0x4f : external Vcm buffer + some unknown parameter

Regards,

Alvin

Hi Marc,

Good to know it works. Thanks for the updating.

Thanks for the update. Please refer to ad9695 datasheet for now, the datasheet of ad9697 is in the queue waiting to be edited. It's recommended to write 0x18e3[6] first, then write 0x18e3[5:0] as I attached earlier, the input common mode voltage will slightly change based on the buffer current settings.

Reg 0x18e3 -> 0x40 :

Reg 0x18e3 -> 0x4f : external Vcm buffer + some unknown parameter

Regards,

Alvin

Sorry, I have been off for few days.
I would like to thank you very much for your help again !

best regards

Marc