ADC3642: Reduced output range in single-ended mode

Kevin TOUTAIN

Part Number: ADC3642

Tool/software:

Hello !

I'm stil working with the ADC3642 and I have some interrogation about the input range of the ADC as working on single-ended mode.

We use a input signal range range 0.3V to 1.5V but the results we got are ... a bit strange. Maybe I miss something.

0.0V --> ADC Code = 2 384
1.5V --> ADC Code = 11 903
1.8V --> ADC Code = 14 000

I configured the ADC registers as following (for REFBUF External set to 1V):

Reg 0x07 - Value 0xA9
Reg 0x0A - Value 0x23
Reg 0x0B - Value 0x46
Reg 0x0C - Value 0xF0
Reg 0x0E - Value 0x0B (REF CTL = '1' and REFSEL = "01" --> External voltage reference using internal reference buffer)
Reg 0x11 - Value 0x35
Reg 0x14 - Value 0x10
Reg 0x15 - Value 0x00
Reg 0x16 - Value 0x00
Reg 0x1B - Value 0x90
Reg 0x1F - Value 0x90
Reg 0x8F - Value 0x02
Reg 0x92 - Value 0x02

(when I use REFBUF internal set to 1.2V, value 0x09 is applied to register 0x0E).

I checked my configuration by reading ADC registers and everything is fine.

I tried to change the REFBUF voltage pin by using either the 1.2V internal or a 1V external and none of them changed the behavior as expected.

(NB: 8-bit MSB are used on figure above)

If I understand correctly, my signal should saturate (0 or 16383DL) when outside the range [Vcm-0.56 ; Vcm+0.56] (regarding the Datasheet page 29) and Vcm is set to 0.9V (measured with a multimeter). So ...

Voltage < 0.9 - 0.56 ==> < 0.34V ==> ADC Code = 0
Voltage > 0.9 + 0.56 ==> > 1.46V ==> ADC Code = 16383

It seems the output value range from the ADC3642 is limited.

Can you please help me to get the full range ?

Thanks in advance.

Regards.

K. TOUTAIN

7 months ago

0 Rob Reeder 7 months ago

TI__Mastermind 38970 points

Hi Kevin,

Can you also please provide a schematic, so we know how the ADC is configured for a single-ended input?

Thanks,

Rob

0 Kevin TOUTAIN 7 months ago in reply to Rob Reeder

Prodigy 20 points

Hello Rob,

Thanks for your answer.

Please find below the schematic of the ADC on our Board.

ADC1_Clock is around 12.5MHz.
Reset_ADC is managed by our software.

Thanks in advance.

0 Rob Reeder 7 months ago in reply to Kevin TOUTAIN

TI__Mastermind 38970 points

Hi Kevin,

What is providing the common mode voltage bias for the driven positive analog input pins?

Are you DC coupled to the previous analog input stage? Amplifier perhaps?

Keep in mind the input fullscale is now reduced by half or 1.125VppFS.

See section 8.3.1.2.2 Single Ended Input of the datasheet.

Thanks,

Rob

0 Kevin TOUTAIN 7 months ago in reply to Rob Reeder

Prodigy 20 points

Hello Rob,

Thanks for your answer.

The positive analog input pin is directly connected to VCM pin from the ADC pin 8.
Yes, we use amplifier in the analog input stage.

I understand that the input fullscale is now 1.125V but I don't understand why the output ADC codes are not on the full possible range (0 to 16383). From our measurements, we don't see the 1.125V range and the fact that the output code seems limited is quite strange (70% FS: 2384 to 14000 are the available codes).

I didn't mentionned it earlier, but when I set the Test Pattern Mode, I get the correct data 0 to 16383 in ADC output.

Thanks.

Kévin

0 Rob Reeder 6 months ago in reply to Kevin TOUTAIN

TI__Mastermind 38970 points

Hi Kevin,

Thanks for the additional info. Let me check on this and see if there is anything missing in your reg write sequence.

Give me a few days to get back to you.

Regards,

Rob

+1 Rob Reeder 6 months ago in reply to Rob Reeder

TI__Mastermind 38970 points

Hi Kevin,

We tested this in the lab today. See attached config file and data captures that show single-ended performance.

Regards,

Rob

ADC3642_AIN-SE_config.zip

0 Kevin TOUTAIN 6 months ago in reply to Rob Reeder

Prodigy 20 points

Hi Rob,

Thanks for the files.

If I correctly understand the pictures you sent me, the ADC3642, in single-ended mode, won't provide codes lower than 4579 or higher than 11816, am I right ?

Thanks in advance.

Kévin

0 Rob Reeder 6 months ago in reply to Kevin TOUTAIN

TI__Mastermind 38970 points

Hi Kevin,

The codes above and below the two values you stated above may show up but won't be valid, as you will be over-ranging the ADC.

You need to bound the codes by half fullscale.

Regards,

Rob

0 Kevin TOUTAIN 6 months ago in reply to Rob Reeder

Prodigy 20 points

Hello Rob,

Thanks for the explanations, I understand much better now.

Have a nice day and thanks again.

Kévin

0 Rob Reeder 6 months ago in reply to Kevin TOUTAIN

TI__Mastermind 38970 points

No problem Kevin, reach out with any other questions.

Regards,

Rob

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ADC3642: Reduced output range in single-ended mode