Communications channels used to be a challenging
exercise in pure analog design. Today,
modulation occurs in the digital domain in many
systems. But the transmitted signal is analog, so
there’s always a conversion.
For any communications system, choices for the digital-
to-analog converter (DAC) and its current-to-voltageconverting
op amp depend on the required bandwidth.
As DACs and op amps get faster, they move closer to
the transmitting antenna.
The DAC needs to convert digital inputs and settle fast
enough to reproduce the modulated signal. The simplest
option for converting a DAC’s current output to voltage
is to use a single-ended transimpedance circuit. It avoids
DAC compliance problems and gives low distortion.
However, the op amp used for the transimpedance
amplifier needs to slew fast enough to match the DAC’s
output, sink or source its full-scale current, and drive the
load. Additionally, transimpedance compensation must
keep bandwidth wide enough without excessive peaking
or oscillation.
A differential current-to-voltage circuit may provide wider
bandwidth, at the expense of higher noise and distortion.
With the right design choices for the application, highfrequency
operation is definitely within reach.
For applications like video, the analog circuit needs to
drive a terminated coaxial or unshielded twisted-pair cable.
In others, the analog signal drives other circuits. The difference
is in the load. Doubly terminated coax will present
37.5 O or 25 O. Doubly terminated, category 5, unshielded
twisted pair gives a balanced 50 O. Another circuit may be
1k or more, but lower impedances deliver higher speed.
BANDWIDTH AND SPEED
What frequency range is important? That depends on the
carrier and modulation frequencies. Many RF receivers
and some transmitters need a tightly controlled sinusoidal
source of 10.7 MHz. Scrambled 100BaseTX Ethernet
produces important frequencies starting around 10 MHz,
with signal components extending to 120 MHz. Five-level
signaling of 1000BaseT gives a similar spectrum on each
of the four twisted pairs it uses.
Necessary slew rate depends on the highest baseband
frequency and amplitude to be reproduced by the analog
output. For a 100BaseTX MLT-3 transmitted signal within
the specified template, 300 V/µs between 0 and +1 V and 0 and –1 V would do it. For a 10.7-MHz, 2-V sinusoid, the
maximum slew rate is sine_SR(f) := 2pVP, 134 V/µs.
Assuming a DAC conversion rate more than twice the
Nyquist frequency, DAC settling time determines the upper
limit of the DAC’s output frequency range. Op-amp settling
time to 1 LSB also shows an upper bound on output
frequency for an accuracy level.
Op-amp settling times are usually specified from a large
input step to 0.1%, 0.01%, and, rarely, 0.001% at noninverting
unity gain. These percentages correspond roughly
to 10-, 13-, and 16-bit LSBs. Performance to unspecified
levels at different gains may be approximated from
typical performance graphs, but there’s no substitute for
testing on the bench once an initial choice is made.
SPECTRAL PURITY, NOISE, AND RESOLUTION
Next, consider harmonic distortion, which can be specified
in several ways. The most common for op amps are
second- and third-harmonic levels below fundamental,
expressed as dBc (dB below carrier). Second and third
harmonics are used because they’re usually the largest.
DAC distortion is also specified a number of different
ways. The most useful for an application with a large frequency
range is spurious-free dynamic range (SFDR) to
Nyquist. This is the ratio of rms signal to rms peak spurious
spectral content up to the Nyquist frequency. SFDR specified
in a frequency band is more important to synthesizing
a strong single tone for a narrowband transmitter.
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