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REF6241: ESR requirements

Part Number: REF6241
Other Parts Discussed in Thread: REF3040, OPA350, DDC112, REF3140,


The DDC112 device datasheet recommends using a voltage reference consisting of 1x REF3040 followed by 1x  OPA350 to the voltage reference input pin for each Delta Sigma ADC in these devices. These devices also use the voltage reference to reset the charge integration capacitor. The reference must be pretty stable and the bypass cap at the reference pin must be charged pretty quick by the OPA350.

I asked on the optical sensors forum if I can drive 4x DDC112 devices with one such a REF3040 reference.  discouraged me from doing so mainly because the DDC112 are older devices that have not being tested/characterized with the newer REF6x41 voltage references.

I plan to experiment on my first prototype. I am going to follow Praveen's suggestion and provide each of the 4 DDC112 devices with its own REF3140 + OPA350 and low ESR bypass capacitors.

But I also want to add 1x REF6241 (with integrated buffer) that I can use to drive the reference of all 4 DDC112 devices simultaneously. Just to test them. I will use jumpers to either only enable the  4x REF3140 references or the 1x REF6241 circuit.

The circuit can be seen in this pdf.

My question is about the requirement of the REF6241 to see an ESR between (~20 milli ohm and 100 milli ohm) @ Capacitance between 10uF to 45uF as can be seen on page 23 of its datasheet.

What values should I choose the PCB track resistances and the ESR of the caps to have a over all Capacitance and ESR to fall in the ranges specified?

I can maybe design the first piece of pcb track to have a ESR of 20 milli ohms and the rest of the track just 1 milli ohm and choose the caps to also have ESRs of 5 milli ohm. Then the reference will see roughly the impedance of the first track together with the combined capacitance value ~ 40uF.

I can also simulate the circuit if I can get an un-encrypted spice model for the REF6241 and maybe something like the AD8881 spice model.

Any suggestions are welcome.


  • Hi Christo,

    I don't think I will be able to provide you with PCB track resistance, that is more of a spice optimizer problem and not a device/application issue.

    In your application I can offer some advice or at least pose some questions. The idea of using a REF3140 + OPA350 is correct but the OPA350 is there for better performance and the ability to drive the 4 devices without affection the REF3140 regulation. I don't think its worth using 1 OPA/REF3140 per reference when a REF3140+OPA350 can power all 4. The max current draw is 275uA with 4 devices it barely hits 1mA. That is easily drivable . I could understand some concerns over having the REF3140 doing it but the OPA350 should handle it especially with a filter after the OPA350.

    Can you link the E2E the other E2E thread?

    The REF6241 oddly connected. Why not have the REF6241 go to all 4 devices in parallel instead of having this daisy chain configuration? I think if you parallel the outputs, it would simplify your problem drastically.