How to clean a power rail for sensitive ADCs - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: September 27, 2020, 07:21:13 pm
content: I am making a power management circuit for multiple sensitive ADCs that will get power from it.The basic idea is kind of like this:Where this circuit I'm about to make will be a power hub of 8 power sensitive ADC. The ADC circuit already has its own filters designed as recommended by it datasheet (PSRR 60dB@100kHz and 90dB@100Hz) which should be theoretically enough, but since I don't have much control and information on how clean the main supply is, it maybe total trash or really good.Hence I would like to place something on the middle. The ADC circuits need 5V, hence I need a regulator and I found this which is advertised to be good at noise sensitive applicationTPS7A47. Is there something I can do to further give better protection against noise from the main supply? Also for inputs above 5V there is nothing to worry since the PSRR would kick in but for inputs that are exactly 5V (and output 5V) from what I know an LDO does not have PSRR in these situations so how can solve this issue?

author: bson
author_rank: Unknown
timestamp: September 27, 2020, 08:28:10 pm
content: The PSRR data assumes a minimum supply voltage headroom. Obviously if you have a 5V supply and 5V outputs, if the supply drops to 4.99V so will the output. Hence without headroom you have (practically) zero PSRR. Same for ADC inputs and reference voltages (which is why deriving it using a resistive divider from the supplies is a bad idea).

author: Kleinstein
author_rank: Super Contributor
timestamp: September 27, 2020, 08:36:57 pm
content: For the LDO to work, it need a little more than the 5 it is supposed to give out. As the PSRR of the usual LDOs gets worse to higher frequencies, it may be a good idea to have some LC or RC filtering between the DCDC and the LDO.Switched mode ACDC converters produce an injected common mode signal. One may want a common mode choke at the ACDC. In theory this could be on either side. How much is needed depends if the is a good ground return path for the CM signal.

author: MasterT
author_rank: Frequent Contributor
timestamp: September 27, 2020, 09:31:25 pm
content: A lot depends on ADC architecture, Sigma Delta doesn't care much about power purity, same time high speed SAR extremely sensitive. Have to use transformer + linear regulators for one of my AD7988.

author: drakejest
author_rank: Regular Contributor
timestamp: September 28, 2020, 04:34:42 am
content: Quote from: bson on September 27, 2020, 08:28:10 pmYes Thats why i would like to not rely on the LDO for cleaning the rail currently where reading stuff(and asking on different forums) lead me circuit looks like thisA ferrite + RC.Quote from: Kleinstein on September 27, 2020, 08:36:57 pmI Am inexperienced when in comes to chokes, and do not how to pick the proper one, Indigikeyaside from the voltage and current ratings it would seem that Impedence @ frequency and DC resistance are the important characteristics. When choosing these value is lower the better ?would a circuit like this good enough ?Quote from: MasterT on September 27, 2020, 09:31:25 pmMy ADC is a 24bit - Delta Sigma , so are you saying that im overthinking things and what i am planning to do is unnecessary?

author: MasterT
author_rank: Frequent Contributor
timestamp: September 28, 2020, 05:16:08 am
content: Quote from: drakejest on September 28, 2020, 04:34:42 amRight. The only things to worry is a voltage reference. If ADC has internal - good, if external , than look into DS for voltage reference, if there is some manufacturer recommendations.

author: drakejest
author_rank: Regular Contributor
timestamp: September 28, 2020, 06:10:36 am
content: Quote from: blueskull on September 28, 2020, 05:08:16 amThe most probable supply voltages will be 5, 12, 24 as those are the most common wall adaptors. The main supply is actually out of the scope of the circuit as it would not fit anymore in the space (AC/DC tend to have large components).Quote from: MasterT on September 28, 2020, 05:16:08 amIt have both, and i made it so to only use internal reference, that way there are fewer components and to prevent me from making a bad reference.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2020, 09:29:18 am
content: If one has to start with 5 V at the input, I would consider lowering the the voltage to the ADC to something like 4.5-4.7 V, so one can still have a working LDO. The SD ADC are quite good in suppressing supply variations, but they are not perfect and also the internal reference gets better when starting from a stable voltage.For the CM chope you probably want a rather high inductance / impedance without getting too much DC resistance and thus voltage lost.

author: exe
author_rank: Unknown
timestamp: September 28, 2020, 09:48:55 am
content: I think a capacitive multiplier will beat most if not all LDOs in terms on high-freq rejection. It does need an ldo after that because it has poor regulation.Most ADCs have 2.7-5.5V supply range, so, feeding it less than 5V usually not a problem as long as it is above its reference voltage (check datasheet how much headroom it needs).

author: drakejest
author_rank: Regular Contributor
timestamp: September 28, 2020, 01:27:25 pm
content: Quote from: Kleinstein on September 28, 2020, 09:29:18 amThe adc actually runs on 3.3v so it has plenty of headroom, dropping a few volts on the RC should not be a problem. So you are saying that its good to still have the choke, how would i build the choke circuit ? Just as simple as this ?

author: drakejest
author_rank: Regular Contributor
timestamp: September 28, 2020, 01:37:34 pm
content: Quote from: exe on September 28, 2020, 09:48:55 amI have seen eevblog video about how great it is, though i am skeptical why i dont see it as much if it is so great. what is its "catch" or has it been replaced with a better technique?

author: exe
author_rank: Unknown
timestamp: September 28, 2020, 02:19:55 pm
content: Quote from: drakejest on September 28, 2020, 01:37:34 pmI don't think there is a "catch". It's a circuit, and it's well described in literature. I can mention a few reasons why it might not be practical or needed: 1) most circuits don't need clean power 2) it takes PCB real estate 3) increases BOM 4) reduces voltage headroom by at least 0.6V 5) takes time to design 6) potentially extra heat dissipation 7) increased rail startup time as the capacitor is charging.