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.

Rev.0, 10/08, WK

Page 1 of 5

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.

VINB

+

-

TRACK MODE VS. HOLD MODE

INPUT FREQUENCY

A

VINA

CP

CP

S1

S2

S3

S4

S6

S5

CH

5pF

CH

5pF

SWITCHES SHOWN IN TRACK MODE

S7