ADC Input Track Layout

[geo_leeman]

Hi all,

I'm working on a project that uses the MAX11040K 24-bit ADC https://www.maximintegrated.com/en/products/analog/data-converters/analog-to-digital-converters/MAX11040K.html]([url]https://www.maximintegrated.com/en/products/analog/data-converters/analog-to-digital-converters/MAX11040K.html)[/url]. I have some sensors that are driven with a 250 Hz square wave and I read their output voltage on the high and low segments to determine their value (that's a whole other story). The sensors output goes though a LMC6482 as a buffer, then to the ADC. My question is do I need to worry about impedance control on the ADC tracks? The buffer should drive the ADC just fine without much thought to this right? After much googling I didn't find much design advice for the input traces, just lots of information about routing the digital and analog separately/at right angles, etc.

* ADC Input Impedance ~ 130k
* LMC6482 Output Impedance ~ a few hundred ohms?

Thanks!

[Yansi]

Worry about what? Track impedance at 250Hz soft-transitioned analog signal?  Really no.


You should be well more worried about your grounding system on the PCB, or using the differential inputs from the ADC correctly. Otherwise, with a bad layout and a few sources of EMI you can easily reduce the effective resolution you could get from the adc to below16 bits in some extremes due to induced noise.

[nctnico]

A 24bit ADC which has no ENOB specification should make alarm bells go off. Also using an external buffer will probably do more harm than good (offset and non-linearity). Did you look at TI's offerings? TI has 24bit ADCs with an internal buffer which can also be taken into account when the device self-calibrates.

When it comes to routing the signals: make sure there is no current flowing in the traces between the ADC what is being measured.

[JPortici]

A 24bit ADC which has no ENOB specification should make alarm bells go off.

which is also a maxim part. or has thing improved recently?

[geo_leeman]

Worry about what? Track impedance at 250Hz soft-transitioned analog signal?  Really no.


You should be well more worried about your grounding system on the PCB, or using the differential inputs from the ADC correctly. Otherwise, with a bad layout and a few sources of EMI you can easily reduce the effective resolution you could get from the adc to below16 bits in some extremes due to induced noise.

Thanks for the input. I have Rev.A of the board and don't really have any signal integrity issues, so that's a good sign. The measurements have been rock solid.

A 24bit ADC which has no ENOB specification should make alarm bells go off. Also using an external buffer will probably do more harm than good (offset and non-linearity). Did you look at TI's offerings? TI has 24bit ADCs with an internal buffer which can also be taken into account when the device self-calibrates.

When it comes to routing the signals: make sure there is no current flowing in the traces between the ADC what is being measured.

This isn't an ideal part, but was the only thing I could find on the market that did what I need - many kHz sampling of 4 channels at 24-bits. As for the buffers - I have to provide a very high impedance input for the sensors. They are electrochemical and any dc bias/current will eventually harm them (hence the square wave driving and buffers).