10-Bit, 20Msps ADC

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13

octal D-latched flip-flop (74ALS374), or use octal

buffers such as the 74ALS541.

Typical Application Using an

External Reference

Figure 7 depicts an application circuit that shuts down

the internal reference, allowing an external reference to

be used for selecting a different common-mode volt-

age. This added flexibility also allows for ratiometric

conversions, as well as for calibration.

Using Transformer Coupling

A small transformer (Figure 8) provides isolation and

AC-coupling to the ADC’s input. Connecting the trans-

former’s center tap to CML provides a +2.25VDC level

shift to the input. Transformer coupling reduces the

need for high-speed op amps, thereby reducing cost.

Although a 1:1 transformer is shown, a step-up trans-

former may be selected to reduce the drive require-

ments.

Single-Ended DC-Coupled Input Signal

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

tion. The MAX4108 quad op amp provides high speed,

high bandwidth, low noise, and low distortion to main-

tain the integrity of the input signal.

4.5k

CMLP

INP

INN

CMLN

4.5k

TO T/H INPUT

5.5k

5.5k

Figure 6. Analog Input DC Bias Circuit

Figure 7. Using an External Reference for REFP, REFN, and CML (internal reference shut down)

MAX1425

REFIN

REFN

R

50

Ω

R

R

R

R

+1V

R

50

Ω

50

Ω

R

R

2

CML

0.1

µF

0.1

µF

0.1

µF

AGND

2

2

4

4

MAX4284

MAX4284

( )

2

- 1V

(

)

REFP

2

+ 1V

(

)