IEEE Solid-States Circuits Magazine - Winter 2023 - 13

The other common scenario is
where the ADC data were never intended
to come off-chip. Ideally, you
have provisions to bypass the normal
signal chain and pipe the data
straight to the output. But unless
you're expecting to deal with raw
ADC or DAC data, the chip outputs
are probably not up to the job. In
this case, the decimator can, once
again, come to your rescue.
Can the data be exported off-chip
with a really slow interface, like SPI
or I2C? In theory, they could, but it
might be painful, and it might require
more stability in the clock and
signal sources because you would
be looking at decimation factors in
the thousands. If circumstances force
the use of an extremely slow interface,
you might be better served by
an on-chip capture memory than
decimation, although all the benefits
of the latter would not be available
(see " The Capture Memory Option " ).
Results, Now!
The ability to get measurement results
immediately can be more helpful
than you might think. Particularly
when debugging brand-new silicon,
the chances of some sort of board or
socket problem are extremely high. The
first thing to do when you encounter
problems with a new ADC is to check
the board and the cabling:
■ Flexing the board (slightly!) can reveal
loose and broken connections.
■ If there is a socket, pressure on
the lid or reseating the chip can
expose flakey contacts.
■ A blast with a heat gun or freeze spray
will alert you to temperature issues.
■ A simple change in the power supply
voltage may make a problem
appear or disappear.
■ A scope probe and finger loading
can change coupling and oscillation
(see " Give It the Finger " ).
■ Even wiggling the cables can reveal
problems.
For all these initial tests, it's helpful
to be looking at the performance
while poking and prodding at
the
board. If you have to acquire a data
record, transfer it to a PC, calculate
the FFT, and then display it, subtle
Data Rate Decimator Options
ADC
÷ N
ADC
ADC
ADC
÷ N
÷ N
÷ N
JESD
LVDS
DAC
Glitch Det.
✓ ✓
✓ ✓ ✓
✓ ✓
✓ ✓ ✓
FIGURE 2: Decimation ('N) is a useful tool for diagnosing high-speed ADCs.
Gitch Det.: glitch detector.
cause and effect is much harder to
spot. Plus, you are probably obliged to
put down the scope probe or heat gun
while you type commands into the
PC. It's much better if you can look at
the spectrum or time-domain reconstruction
while you push on the socket
and wiggle the connections. This
can help you converge rapidly on the
source of the problem and get your
colleagues in off the window ledge
quickly. (There is much anticipation
surrounding brand-new silicon. Emotions
run high.)
Even when most of the onion has
been peeled and you're trying to
find the last tiny defect buried deep
within the ADC, it's good to understand
the parameters that make it
appear. Real-time output allows you
to quickly sweep frequency, amplitude,
voltage, bias, and, in a crude
way, temperature so that you can
isolate the cause. These sensitivities
are sometimes the only clues as to
what may be wrong.
The key to real-time output is, once
again, decimation. Slowing down the
data rate allows you to reconstruct
the waveform with a simple DAC, either
on-chip or off. Slow DACs have
amazing spectral purity and are easier
to use than DACs that can keep up
with undecimated high-speed ADCs.
One final advantage of decimated
real-time data is the ability to generate
" crossplots, " as described in " What
We Did for Fun Before the Internet. "
This venerable technique has fallen
into obscurity, but it's nice to know
it's there if you need it. And it does
illustrate the diagnostic power of realtime
results.
Make No Mistake
As discussed in " The Other ADC Test "
[1], you often want to test an ADC at
near-Nyquist speeds, but you don't
THE CAPTURE MEMORY OPTION
On-chip capture memory is a good alternative to decimation for diagnosing high-speed ADCs.
Dumping the ADC output directly into memory avoids the need for exotic I/O and allows you
to read out the data record with any available interface or even do the required analysis onchip.
Adding a large memory block and processing power to an SoC solely for ADC diagnosis
would be prohibitive, but it may already be there for calibration or other signal processing tasks.
Unfortunately, on-chip capture memory cannot provide all the benefits of decimation because
it is still relatively slow. It cannot facilitate real-time output, for example. And the embedded
processor probably cannot process data fast enough to make error rate measurements practical.
IEEE SOLID-STATE CIRCUITS MAGAZINE WINTER 2023
13
JESD Not Reliable
CER Testing
Real-Time Output
Alias Signal

IEEE Solid-States Circuits Magazine - Winter 2023

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Winter 2023