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Hello i am building an ADC circuit, this is my first time building a proper adc that is higher performance. The adc is the ADS131

The ADC will be powered by an isolated 3.3v and I would like to know what would be a good Vref voltage value would 3.3v just be fine. What is the popular convention for this stuff.Where i plan to use this adc is to measure low voltage AC differential signals. The adc is 24 bits and would like make effective use of as many of those 24bits

I am also planning of placing a voltage reference IC but there are several types and do no know which ones are the best. Since the ADC is powered by 3.3v i would most probably need to use a reference voltage below 3.3 because of tiny voltage drop out would that be okay? or i have to find another supply for the Vshunt IC. Is there any notable difference in using a fixed like this one and and adjustable version like this one

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    \$\begingroup\$ More information needed. We don't know what you are using it for, how you are using it, what your ADC is, or how accurate you need it to be. All of which determines how good a Vref you actually need. The question as it stands is saying "I need a vehicle. What is a good one?" Also not sure what virtual grounds have to do with this. \$\endgroup\$
    – DKNguyen
    May 13, 2020 at 21:23
  • \$\begingroup\$ @DKNguyen apologies, I edited the question to add/remove some parts. About the virtual ground i do not know too, Digi-keys filters has a tag for digital ground on voltage reference ICs and there were quite a few of them too. Im not sure if using those would benifit an adc \$\endgroup\$
    – Jake quin
    May 13, 2020 at 21:51
  • \$\begingroup\$ Getting full 24-bits is...tricky. \$\endgroup\$
    – DKNguyen
    May 13, 2020 at 22:07
  • \$\begingroup\$ @DKNguyen with the components i linked or realisticly? I do only expect 18 to 21 effective number of bits. \$\endgroup\$
    – Jake quin
    May 14, 2020 at 0:11
  • \$\begingroup\$ You are measuring AC signals so you mostly care about noise at a given bandwidth of (?) and not so much about DC drift ? \$\endgroup\$ May 14, 2020 at 0:45

3 Answers 3

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The ADC will be powered by an isolated 3.3v and I would like to know what would be a good Vref voltage. The adc is the ADS131

The device uses an internal (fixed) reference voltage: -

enter image description here

There is no facility to use an external voltage reference.

\$\color{blue}{\text{Im sorry, i linked the wrong ADC im using the 8 channel version > ADS13M08}}\$

Well, this is the specification for the allowed operating voltage range for the ADS13M08: -

enter image description here

So, unless you have particularly onerous reference stability requirements that can only be accomplished with an external reference, I would use the internal reference. The only one that is suitable is the MAX6070/1 device as it produces 1.25 volts. However, the worst-case drift from this device and the drift from the internal ADC circuitry will be slightly worse than using the internal reference. The only benefit is that the external reference will have a higher initial accuracy than the internal reference.

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  • \$\begingroup\$ Im sorry, i linked the wrong ADC im using the 8 channel version ADS13M08 my edit to the question is under pending. This ADC has a Vref as well as an internal 1.2Vref. \$\endgroup\$
    – Jake quin
    May 14, 2020 at 9:37
  • \$\begingroup\$ Ohhhh.. well now all the reading i have done seems to look pointless now... that ref voltage is too close to the peak voltage i am reading, would dividing it with resistor be the best choice to read higher voltages? \$\endgroup\$
    – Jake quin
    May 14, 2020 at 10:48
  • \$\begingroup\$ @Jakequin Dividing down the input voltage would appear to be the only course of action. \$\endgroup\$
    – Andy aka
    May 14, 2020 at 11:02
  • \$\begingroup\$ Thank you very much for pointing this detail out good sir this saved me a lot of headache later on. i know that you have said its onerous, but does having an external assuming its really good can earn me a bit or two of effective number of bits thus justify the extra effort? \$\endgroup\$
    – Jake quin
    May 14, 2020 at 11:23
  • \$\begingroup\$ @Jakequin You need to do an error budget calculation AND take into account drift with temperature and noise. \$\endgroup\$
    – Andy aka
    May 14, 2020 at 12:43
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Let us talk about achieving a clean VREF (this example also applies to your analog Vin), in the presence of interferers.

First, we'll do math for (A) magnetic interferers, then (B) power supply interferers, then (C) Electric Field interferers from your MCU, finally (D) the effect of Ground currents.

(A) Suppose your VREF (or Vin) has no ground plane, because you heard planes are not needed at low frequencies, and a VREF is DC.

Suppose there is 1cm by 4cm loop of PCB traces, between your VREF CIRCUIT and the ADC. In other words, the VREF trace is located 1cm away from the GND trace.

Suppose 4cm away is power wiring to the Power Supply of your system. Suppose youe system draws 10 watts (0.1 A from 117 Vac), and the rectifiers have 10X that as peak current, with 1us risetime thru the diodes. Thus your dI/dT will be 1 A / 1 micro_second.

How much trash is coupled into the VREF traces?

Lets used the formula(*)

Vinduce = 2e-7 Henry/meter * (Area / Distance) * di/dT

[if this induced trash is less than 1uv, then we can ignore this source of error.]

Vinduce = [2e-7 * (1cm * 4cm) / 4cm] * 1 A/1 us

Vinduce = 2e-7 * 0.01 * 1e+6

Vinduce = 2e-7 * 1e-2 * 1e+6

Vinduce = 2 * 10^(-7 -2 +6) = 2 * 10^-3 = 2 mV

Thus the computed interference is 2,000X larger than our "Not a problem" threshold.

Thus there is a problem, given the assumptions stated.

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What if you use a Ground plane? will not do much good, for pulses slower than 4 MHz (100 nanoseconds TRise).

You will need thick copper (expensive) or thick aluminum or some steel sheets.

Ever wonder why high-precision equipment is HEAVY? Partly because of the need to SHIELD the circuitry.

(*) the base formula for magnetic coupling between a long straight wire carrying a current with some "dI/dT", into a co-planar rectangular loop of some Area and separated by Distance from the wire, is

Vinduce = [ MUo * MUr * Area/(2 * PI * Distance) ] * dI/dT

Given MUr = 1 for air and FR-4 and copper and aluminum, and MUo = 4 * pi * 1e-7, the formula simplifies to what we use.

For complete accuracy, you can write the integral and find the effect of Natural_Log on the final result. The formula we use has the Distance being the shortest distance between the WIRE and the LOOP, thus overestimates the Vinduce.

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regarding mechanical design: placing such circuitry anywhere near high_current power cables is asking for trouble. Dishwasher motor spikes, microwave_oven rectifier spikes, airconditioner compressor spikes all ride atop the 60Hertz current waveforms.

The mechanical configuration in this math assumes the Reference IC is located 4cm away from the ADC.

(B) regarding Power Supply trash: the 24-bit ADC has 6 * 24 == 144 dB range of resolution, but only 78dB Power Supply Rejection Ratio (PSRR) at 56/60Hertz, already down over 20dB from the DC PSRR spec. Thus we'll assume the ADC is operating on a 10:1 drop in PSRR for 10:1 increase in VDD trash frequency. At 500,000 Hertz (2uS ringing, well below Switch Reg trash), the ADC likely will have ZERO ability to reject trash. ITS YOUR JOB to REMOVE THIS TRASH

LDOs cannot remove high frequency trash; their internal servo-loops cannot function fast enough, even if you spend the Iddq budget and buy a high-current LDO. YOU MUST FILTER THE RAW Power Rail BEFORE THE LDO

For filtering, a Ferrite Bead does nothing. Use a discrete resistor and a discrete capacitor, in the voltage into the LDO. I suggest 100 ohms and 100uF, for a 10 millisecond time constant and 16Hz F3dB. And follow that with 10 ohms and 10uF, (because the first R+C will be limited in far-out attenuation by the ratio of 100 ohms and the ESR+ESL of the 100uf cap). This 2nd R+C has 100uS time constant, or 1,600Hz F3dB, and should provide 60 dB attenuation at 1MHz (about where Switch Reg L+C ringing will occur).

Is this much VDD filtering (all before the LDO), necessary? Perhaps not, because the ADS131 24-bit ADC has 5 microvolt RMS noise anyway.

(C) Electric Field interference

(D) Ground trash (or we could make this thermo-electric errors from temperature changes in the PCB, causing low frequency drift of offset.

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  • \$\begingroup\$ Well atleast i do plan to place this in a full metal enclosure, on a small electrical control box. If i place one of those IC metal shield kinda like how most wifi card IC come in would that offer alot of help ? I noticed you did not mention about V reference IC's (V series/ Vshunts IC's). How significant would their inclusion be? \$\endgroup\$
    – Jake quin
    May 14, 2020 at 6:30
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Texas Instruments made a tips and tricks guide for designs with voltage references, It mainly talks about different types and archetecture of Vref shunts.

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