MAX6226 vref and ground planes - Page 1
Forum section: electronics / general

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 16, 2023, 12:54:05 am
content: Looking for some advice from those with more experience with the MAX6226 voltage reference. This is a 5V LCC package, and the data sheet has some explicit recommendations:QuoteHowever, looking at the MAX6226EVKIT evaluation board, there is a full ground plane on the bottom side and on the top side, there is a ground pour, and ground track passing up the middle to connect the two large islands above and below the chip. That passes through the place where pin 8 is located, so there is both copper and solder resist under that pin (but no electrical connection).It seems to me that their evaluation board doesn't follow the advice in their data sheet. Or, I misunderstood the advice?https://www.mouser.com/new/maxim-integrated/maxim-max6226evkit-evaluation-kit/https://www.mouser.com/new/maxim-integrated/maxim-max6226-voltage-reference/

author: iMo
author_rank: Super Contributor
timestamp: February 16, 2023, 07:55:55 am
content: I think it applies when placing the reference on a larger, complex pcb board, with a lot of devices and AC currents on it which may contribute to those negative factors. The power and ground planes in a large design may carry large AC currents, and the large temperature/humidity changes (or a mechanical stress) may bend the pcb such it "paddles" with the pin8 (perhaps wired to a capacitive sensitive internal node). With a small and static design (talking currents and temperature) as that eval board is, it may play no role..

author: AeroETech
author_rank: Contributor
timestamp: February 16, 2023, 11:57:01 am
content: It is not uncommon for evaluation boards to be drawn up by a pretty inexperienced designer. More than likely, they simply didn't follow the datasheet advise, and no-one caught it.For things like this, follow the datasheet guidance unless it's completely absurd.

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 16, 2023, 06:09:09 pm
content: Makes sense about large vs. small boards having different constraints.In my application, I plan to have a tiny Vref board with just the MAX6226, a dual op-amp (the design needs both +5 and -5 references) and the associated decoupling and noise reduction caps and a couple of low-ppm/C resistors for the inverting op-amp. That will be sandwiched between two identical DAC (AD5781ARUZ) boards. So I think flexing and return currents will be small.Understand too about eval boards not necessarily attracting a lot of design resources, so on my board I still might cut out the ground track running up the top side, also the solder mask under pin 8. But keep the complete ground plane on the bottom side.

author: AeroETech
author_rank: Contributor
timestamp: February 27, 2023, 12:48:03 pm
content: Don't use individual resistors for the inverting amp, use something like a Vishay MPM part. Much better ratio tracking.

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 27, 2023, 06:35:08 pm
content: Thanks for the suggestion. On the face of it that seemsa better part: ratio tracking of 0.05% (abs tolerance 0.1%) and ±2 ppm/°C (typical) tracking TCR.However practical considerations preclude it: Mouser has zero stock, 40 weeks lead time, and MOQ 500 means the investment would be $1,270.00 which is a little high for a one-off hobbyist project.So I'm settling fortwo Susumu RG2012V-103-P-T1which have abs tolerance of 0.02% and ±5 ppm/°C (typical) TCR. More importantly Mouser has more than 1000 in stock, they cost $2.44 each for 10 and I already have 20 of them for another project, so can easily spare 2.

author: ivo
author_rank: Regular Contributor
timestamp: February 28, 2023, 06:19:54 am
content: Relying on Conrad Hoffman's advice fromhis famous series, he demonstrated that it is possible, with your own use of time, and empirical testing, instead of trying tobuytempco tracking, it is far easier (or... takes less time than 47 weeks) to test+select two resistors out of many that having matching tempco. If you have no ability to digitally calibrate, then you need to select for both value matching and tempco tracking, but if you have the ability to digitally calibrate offsets, you only need to match tempco. Although funnily enough this is probably far easier to do with THT parts.

author: AeroETech
author_rank: Contributor
timestamp: February 28, 2023, 08:54:07 am
content: Mouser and Digikey both have several variants of them in stock, available from about 4 bucks in single piece quantities.https://www.mouser.com/c/passive-components/resistors/resistor-networks-arrays/?series=MPM%7C~MPMT&instock=yThey are almost always much more accurate than datasheet specifications. Here's one used for op-amp gain in a -0.01V/V gain configuration. -1.25V reference voltage as input, 12.5mV output.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2023, 05:52:46 pm
content: Resistor arrays are often better than the specs when it comes to tracking, but similar single resistors from the same batch are also usually matching quite a bit better than there individual specs.Given the pricing of the AD5781, there is no need to skimp on the resistors and reference.The AD5781 actually already includs precision resistors to invert the reference. So it would make relatively little sense to use inferior external resistors instead.

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 28, 2023, 08:02:02 pm
content: QuoteOh, I know about those. But these aren't to invert the output; they are to create the negative Vref as this DAC needs both a positive and a negative reference and in my application, I need a DAC swing from +5V to -5V.The DAC output goes to a simple non-inverting buffer.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2023, 08:11:28 pm
content: The DAC output range goes from VrefN to VrefP. So with a +-5 V reference it would cover the +-5 V range. It can still be somewhat challanging to use the seprate reference drive and sense pins of the ADC together with the inverter.

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 28, 2023, 08:47:54 pm
content: QuoteYes, exactly.

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 28, 2023, 09:35:39 pm
content: QuoteMaybe I am not understanding a problem you are seeing, but the force and sense pins are being used like this in my design, which closely follows the datasheet (this is a snapshot of work in progress; the board has not been finalized and ordered yet):DAC board schematic (PDF)One dual op-amp is used to drive the positive and negative reference inputs, the other is used to give two buffered outputs. There are two such DAC boards, sharing a common voltage reference board:Vref board schematic (PDF)