Page 143 - Troubleshooting Analog Circuits

I30 IO. The Analog/Digital Boundary

With ADCs, Paper Designs Aren’t Adequate

On one 10-bit ADC I designed, when the customer found some problems that I
couldn’t duplicate in my lab, I bought one plane ticket for me and one for my best
scope. After a few hours we arrived at the scene, and in less than an hour I had the
problem defined: The customer expected our converter to meet all specs with as
much as 0.2 V DC plus 0.2 V AC, at frequencies as high as 5 MHz, between the
analog ground and the digital ground! ! Outrageous! ! Amazingly, our architecture
was such that by deleting one resistor and adding one capacitor, I could comply with
the customer’s wishes. Most ADCs couldn’t have been adapted to work-the cus-
tomer was fantastically lucky that I had used a weird design that was amenable to this
modification. My design was a high-speed integrating converter with an input
voltage-to-current converter that just happened to be capable of rejecting wideband
noise and DC offsets between grounds.
The general lesson is that any ADC system is nontrivial and should be engineered
by actually plugging in some converter circuits. “Paper designs” usually don’t hold
water in ADC systems.
To beat the requirement that every ADC have its own set of power supplies dedi-
cated exclusively to powering just the single converter, you may want to bring power
to your PC boards in unregulated or crudely regulated form, and then put a small reg-
ulator right near each ADC. These small regulators (whether LM320L15, kA78L05,
LM3 17L, or whatever) do not have a high power-supply rejection ratio at high fre-
quencies. You can resolve that problem with decoupling, so you have a chance to
make the scheme work. I hasten to point out, however, that I haven’t actually built
such a system myself very often.

Don’t Let Ground Loops Knock You for a Loop
The need for multiple (separate) power supplies, or at least multiple regulators,
comes, of course, from the many paths taken by ground currents flowing to and from
the power supplies. If you don’t keep these paths scrupulously separate, the ground
loops can cause bad crosstalk between various parts of the system-low-level analog,
high-power analog, and digital. So be very careful to avoid ground loops when you
can. Although the electrical engineering faculty at your local university might not
agree, a general solution to the ground-loop problem would be an excellent subject
for a PhD thesis. If you write such a dissertation, please don’t forget to mail me a copy.
Some successive-approximation ADCs have separate buffers feeding their output
pins, but other designs try to save money, parts, power, or space by using the internal
registers to drive both the internal DAC and the output pins. In this case, external
loads on the outputs can cause poor settling and noise and can thus degrade the per-
formance of the converter. If you’re using ADCs, you should find out if the outputs
are connected directly to the DAC. Sometimes, as previously mentioned, a preload
on each bit output can help to accelerate settling of an ADC’s internal DAC. After
all, TTL outputs must be able to drive more current than their DC specs state-they
have to meet their AC specs.

VFCs and FVCs Frequently Find Favor

The voltage-to-frequency converter (VFC) is a popular form of ADC, especially
when you need isolation between the analog input and digital outputs. You can easily
feed a VFC’s output pulse train through an optocoupler to achieve isolation between