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. Praveen Aroul 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. www.dropbox.com/.../REF3140.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.