MT-074 Datasheet, PDF (1/5 Pages) Analog Devices – Differential Drivers for Precision ADCs

MT-074
TUTORIAL
Differential Drivers for Precision ADCs
DIFFERENTIAL INPUT ADC CHARACTERISTICS
Many high performance ADCs are now being designed with differential inputs. A fully
differential ADC design offers the advantages of good common-mode rejection, reduction in
second-order distortion products, and simplified dc trim algorithms. Although they can be driven
single-ended, a fully differential driver usually optimizes overall performance.
One of the most common ways to drive a differential input ADC is with a transformer. However,
there are many applications where the ADCs cannot be driven with transformers because the
frequency response must extend to dc. In these cases, differential drivers are required. This
tutorial focuses on driving high resolution 16- to 18-bit ADCs with sampling rates up to 10
MSPS. The bandwidth of the input signals is generally limited to a few MHz. Tutorial MT-075
discusses differential amplifiers suitable for driving higher speed ADCs.
Most high performance CMOS switched capacitor pipelined ADCs have differential inputs.
similar to that shown in Figure 1.
SWITCHES SHOWN IN TRACK MODE
S5
VINA
Z_IN
VINB
CP
S1
S2
CP
CH
5pF
S7
CH
5pF
S3
+
A
-
S4
S6
Z_IN IS A FUNCTION OF:
‹ TRACK MODE VS. HOLD MODE
‹ INPUT FREQUENCY
Figure 1: Simplified Input Circuit for a Typical Unbuffered Switched
Capacitor CMOS Sample-and-Hold
The differential structure is typically carried through most of the ADC. This makes matching
requirements easier as well as reduces second-order products. In addition, the differential
structure helps in common-mode noise rejection.
Rev.0, 10/08, WK
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