輸入魔法——差分信號允許輸入擺幅超過電源電壓
Input Magic—Differential Signals Allow Input Swing to Exceed
Supply Voltage
Q: My ADC has a 1.8-V power supply. How can it
have a 2-V p-p input range?
A: The latest generation of low-power, high-speed
ADCs—manufactured on fine-line silicon processes—run on low-voltage supplies.
ADC designers face a tradeoff between making the input range bigger to get
better signal-to-noise ratio (larger signals provide higher SNR), and making the
input range smaller to ease the drive requirements. Over the years we’ve seen
ADCs with 5‑V supplies and 4-V p-p input ranges, and 3-V supplies with 2-V p-p
input ranges, but these didn’t raise any eyebrows. Over the past few years,
however, we’ve seen a host of ADCs with 1.8-V supplies and 2‑V p-p input ranges.
These raise some seemingly reasonable questions: How can the ADC have a 2-V p-p
input voltage range while running on 1.8-V supplies? Doesn’t this require the
signal to exceed the supply rails?
The overlooked detail, of course, is that the analog input signal
to most high-speed ADCs is differential. Transmitted as complementary
single-ended signal pairs on the VIN+ and VIN– pins, the differential input signal VINDIFF = VIN+ – VIN–. The single-ended components, centered within the supply
rails, swing only half the amplitude of the differential signal, with a typical
common-mode voltage of VSUPPLY/2.
Differential signals are advantageous because they provide good
common-mode rejection and inherent cancellation of even-order distortion (This
is only true if you have perfect amplitude and phase matching, but that’s
another story.). An often overlooked advantage of differential signals, however,
is that the amplitude of a differential signal can have twice the amplitude as a
single-ended signal within a given supply range. As ADC designs move to even
lower supply voltages, the headroom for the input signal will continue to be
squeezed, and the differential signal will occupy more of the available supply
range. In dc-coupled applications, the common-mode voltage of these low-voltage
ADCs presents an interface challenge to the drive amplifiers, but for many
applications the signal can be ac-coupled to the ADC, so this will not be an
issue.
I invite you to comment on Differential Signals in the Analog Dialogue Community on EngineerZone.
References
Analog Dialogue
Ardizzoni, John and Jonathan Pearson, “Rules of the Road” for High-Speed Differential ADC Drivers, Analog Dialogue, Volume 43, Number 2, 2009
Ardizzoni, John and Jonathan Pearson, “Rules of the Road” for High-Speed Differential ADC Drivers, Analog Dialogue, Volume 43, Number 2, 2009
Application Notes
AN-1026: High Speed Differential ADC Driver Design Considerations
AN-1026: High Speed Differential ADC Driver Design Considerations
RAQs
Driving Miss ADC
Driving Miss ADC
Tutorials
MT-075: Differential Drivers for High Speed ADCs Overview
MT-074: Differential Drivers for Precision ADCs
MT-075: Differential Drivers for High Speed ADCs Overview
MT-074: Differential Drivers for Precision ADCs
Webcasts
The Latest on Driving ADCs Differentially: Part 2
The Latest on Driving ADCs Differentially: Part 1
The Latest on Driving ADCs Differentially: Part 2
The Latest on Driving ADCs Differentially: Part 1
Author | |
David Buchanan [david.buchanan@analog.com] received a BSEE from the University of Virginia in 1987. Employed in marketing and applications engineering roles by Analog Devices, Adaptec, and STMicroelectronics, he has experience with a variety of high-performance analog semiconductor products. He is currently a senior applications engineer with ADI’s High Speed Converters product line in Greensboro, North Carolina. |
Submit your question to: www.analog.com/askdavid
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