12-Bit, 40Msps, +3.3V, Low-Power ADC

with Internal Reference

______________________________________________________________________________________

13

Using Transformer Coupling

An RF transformer (Figure 8) provides an excellent solu-

tion to convert a single-ended signal to a fully differen-

tial signal, required by the MAX1421 for optimum

performance. Connecting the center tap of the trans-

the input. Although a 1:1 transformer is shown, a 1:2 or

1:4 step-up transformer may be selected to reduce the

drive requirements.

In general, the MAX1421 provides better SFDR and

THD with fully differential input signals over single-

ended input signals, especially for very high input fre-

quencies. In differential input mode, even-order

harmonics are suppressed and each of the inputs

requires only half the signal swing compared to single-

ended mode.

Single-Ended AC-Coupled Input Signal

Figure 9 shows an AC-coupled, single-ended applica-

tion, using a MAX4108 op amp. This configuration pro-

vides high-speed, high-bandwidth, low noise, and low

distortion to maintain the integrity of the input signal.

Grounding, Bypassing, and

Board Layout

The MAX1421 requires high-speed board layout design

techniques. Locate all bypass capacitors as close to

the device as possible, preferably on the same side of

the board as the ADC, using surface-mount devices for

minimum inductance. Bypass REFP, REFN, REFIN, and

CML with a parallel network of 0.22µF capacitors and

network of a 10µF bipolar capacitor in parallel with two

ceramic capacitors of 1nF and 0.1µF. Follow the same

Multilayer boards with separate ground and power

planes produce the highest level of signal integrity.

Consider the use of a split ground plane arrangement

to match the physical location of the analog ground

(AGND) and the digital output driver ground (DGND) on

the ADCs package. The two ground planes should be

joined at a single point so that the noisy digital ground

currents do not interfere with the analog ground plane.

Alternatively, all ground pins could share the same

ground plane, if the ground plane is sufficiently isolated

from any noisy, digital systems ground plane (e.g.,

downstream output buffer, DSP ground plane). Route

high-speed digital signal traces away from sensitive

analog traces and remove digital ground and power

planes from underneath digital outputs. Keep all signal

lines short and free of 90 degree turns.

Static Parameter Definitions

Integral Nonlinearity (INL)

Integral nonlinearity is the deviation of the values on an

actual transfer function from a straight line. This straight-

line can be either a best straight-line fit or a line drawn

between the endpoints of the transfer function, once off-

set and gain errors have been nullified. The static lin-

earity parameters for the MAX1421 are measured using

the best straight-line fit method.

Differential Nonlinearity (DNL)

Differential nonlinearity is the difference between an

actual step-width and the ideal value of 1LSB. A DNL

N - 7

N

N - 6

N + 1

N - 5

N + 2

N - 4

N + 3

N - 3

N + 4

N - 2

N + 5

N - 1

N

N + 6

7 CLOCK-CYCLE LATENCY

ANALOG INPUT

DATA OUTPUT

CLK

CLK

Figure 6. System and Output Timing Diagram