[Design View / Design Solution]
Designing For High Speed In Current-To-Voltage Conversion
As new communications systems reduce the number of RF up-conversions, design of the digital-to-analog stage becomes more challenging.
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.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
