ACD Reference Voltages

[rayhall]

I had a design house do the schematic for my project. The design as two reference voltage circuits for the ADC VRH and VRL. To me these are far more complicated then what I have seen elsewhere. The circuits output 0.7v and 4.7v to VRL and VRH.

The voltage inputs have to read 0 to 5 volts. All the analog inputs have a circuit that converts the 0 to 5 volt to 0.7 to 4.7v.  To me I cannot see the point in doing this. What do you think.

I have attached a screen shot of the reference circuits and one of the analog volt inputs.

Ray.

[Derrick · Accepted solution]

Hi Ray,

Wow!  That circuitry is significantly more complicated than any that I’ve seen before for an MCU’s ADC.  You don’t mention exactly which MCU that you are using, but normally you can find a Freescale evaluation board that can be used as a reference.  Ordinarily you would tie VRH directly to VDDA and connect VRL to VSSA.  This is especially true if you need to convert analog signals that are swinging between VSSA and VDDA.  Sometimes, due to various circuit interactions, there could be slight inaccuracies with signals that are at or very close to the power rails, but this can usually be dealt with by conditioning the input signals if required.

Be sure to use decoupling capacitors between and close to the VRH and VRL pins.  Again, I recommend using an existing evaluation board’s layout as an example.  If you suspect that your system may have some power supply noise that could impact the ADC accuracy then you might want to consider adding ferrite beads or inductors.  Although it looks like you’re already using a separate analog supply rail (i.e., “+5VA” versus “+5V”) which is “quieter” than the digital supply(?).

Keep the source impedance of the external analog signals as low as possible – nothing over 10k ohms and preferably closer to 1k ohms or lower.  A high source impedance will affect the conversion accuracy.

Best Regards,

Derrick

[rayhall]

Derrick,

Thank you for your input. The whole design is like this. The component count is huge and makes placing them on the PCB next to impossible without making the PCB bigger which is one thing I will not do.

I am using the S12XE.

Yes there is a separate analog supply rail.

It was looking at my evaluation board schematic, that made me believe that this was over the top.

Ray.