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Ryan Andrews

May 22, 2015

powering
delta-sigma ADCs

ADS1262

dc
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ads1278

ADS1220

delta sigma
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Caution! Your ADC may be only as good as its power supply

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When it comes to the analog signal chain, everyone understands the importance
of the input signal path. We design our systems to capture and preserve the
integrity of signals of interest, while doing our best to block out or reduce
interference. We pay careful attention to the selection of every component
placed along the way…and then we just power it.

I once heard someone describe power supplies as “the shoelaces of electrical
circuits.” Like circuits, a lot of hard work goes into the design and style of a
shoe, but nobody thinks about the shoelaces until the end. While power supplies
tend to be more of an afterthought, their design may be just as important as the
signal chain itself.

In the first part of this series, I will introduce the concept of power
supply rejection (PSR) and explain how power supplies can impact the performance
of delta-sigma analog-to-digital converters (ADCs).

My DC power supply is “rock solid”… right?

Believe it or not, your power supply probably isn’t as solid as you think.
From a DC perspective, both component tolerances and temperature drift can cause
your supply output to vary from board to board and over temperature. A slight
change may not seem important if it remains within your ADC’s operating
conditions, but this may produce additional offset and gain error in the ADC
transfer function.

Power supplies can also be quite noisy. Depending on their source or the
surrounding environment, DC power supplies can carry an AC component as well. We
are all familiar with the (figurative) headaches caused by 50 Hz/60 Hz power
lines. Power-line noise often finds a way to couple in through lighting or
nearby equipment, even for battery-powered applications. This noise may fall
directly within your signal bandwidth of interest for some applications.

In addition, more complex systems can generate multiple power supplies from a
single source using a switching DC/DC converter to “buck” or “boost” their
source to another voltage. The frequencies of these switching power supplies may
exist well beyond the signal bandwidth of interest, but these frequencies have
the potential of aliasing back into the passband. This directly affects
important AC specifications such as signal-to-noise ratio (SNR) and
spurious-free dynamic range (SFDR).

ADCs expect a stable and purely DC supply, so any variation in that supply
has the potential to change the ADC’s performance. Like most active components,
ADCs are designed with some inherent immunity to power supply changes, but it is
important to quantify whether this is enough for your application.

How susceptible is my ADC to its power supply?

ADC PSR describes the change in the ADC output relative to the change in its
power supply. Sometimes it is specified as a ratio and referred to as the power
supply rejection ratio (PSRR) expressed in decibels (dB). TI’s delta-sigma ADCs
typically specify PSRR in two ways: PSRRDC and PSRRAC.

PSRRDC measures the ADC’s rejection of a DC shift in the power
supply. To measure PSRRDC, you short the ADC inputs together and bias
them to mid-supply as shown in Figure 1. Then, you power the analog ADC supply
(AVDD) from a clean DC source and record the offset voltage at the output. As
you increase or decrease the supply by 100 mV, record any change you observe in
the ADC output. Figure 2 illustrates how the original offset voltage in the ADS1220 changes by 562 nV with a 100 mV change in supply
voltage. Using Equation 1, PSRRDC is calculated to be -105 dB.

Figure 1: PSRRDC test setup

Figure 2: ADS1220 PSRRDC measurement

Equation 1

PSRRAC measures the ADC’s rejection of power-supply noise or
ripple. To measure PSRRAC, you keep the ADC inputs shorted together
and biased to mid-supply while adding a 100mVPP sine wave
(VIN) to the DC power supply as shown in Figure 3. An AC signal
generator with an offset voltage equal to the desired supply voltage will do the
trick. For the frequencies of interest, you record the amplitude of the tone
seen in the Fast Fourier Transform (FFT) of the output data and compute the
PSRRAC using Equation 2. For the ADS1278, a 100 mV amplitude ripple translates to -28 dB from
full-scale with a 2.5 V reference voltage and 24 bits of resolution. If this
supply ripple has a frequency of 60 Hz, the ADS1278 will attenuate the ripple by another -80 dB,
lowering the output magnitude at 60 Hz to -108 dB. Datasheets usually provide
the PSRRAC specification for 50 Hz and 60 Hz in the electrical
characteristics table and for all other frequencies in a typical characteristics
plot up to 100 kHz or 1 MHz.

Figure 3: PSRRAC test setup

Figure 4: ADS1278
PSRRAC
measurement

Equation 2

As I mentioned before, most ADCs have some level of PSR based on how they are
designed. For those applications that require better PSR, the next post in this
series will cover ways to improve PSR in your system.

In the meantime, check out other posts by my colleagues with design tips and
delta-sigma ADC basics.

Additional resources:

Search TI’s precision ADC portfolio, including ADCs like the ADS1220 and ADS1262 for temperature measurement and programmable logic
controller (PLC) applications. Log in to myTI and download the new e-book, Best of Baker’s Best: Delta-Sigma ADCs. It compiles
the best of the short articles about delta-sigma ADCs by popular EDN
columnist and TI analog applications engineer, Bonnie Baker.

Browse the all-new TI Precision Labs for a refresher on other analog design
topics with over 30 hands-on trainings and lab videos.

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