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

with Internal Reference

______________________________________________________________________________________

11

The MAX1421 provides three modes of reference oper-

ation:

•

Internal reference mode

•

Buffered external reference mode

•

Unbuffered external reference mode

In internal reference mode, the on-chip +2.048V

bandgap reference is active and REFIN, REFP, CML,

and REFN are left floating. For stability purposes,

bypass REFIN, REFP, REFN, and CML with a capacitor

network of 0.22µF, in parallel with a 1nF capacitor to

AGND.

In buffered external reference mode, the reference volt-

age levels can be adjusted externally by applying a

stable and accurate voltage at REFIN.

In unbuffered external reference mode, REFIN is con-

nected to AGND, which deactivates the on-chip buffers

of REFP, CML, and REFN. With their buffers shut down,

these nodes become high impedance and can be dri-

ven by external reference sources, as shown in Figure 3.

Clock Inputs (CLK,

C

CL

LK

K)

The MAX1421’s CLK and CLK inputs accept both sin-

gle-ended and differential input operation, and accept

CMOS-compatible clock signals. If CLK is driven with a

single-ended clock signal, bypass CLK with a 0.1µF

capacitor to AGND. Since the interstage conversion of

the device depends on the repeatability of the rising

and falling edges of the external clock, use a clock with

low jitter and fast rise and fall times (<2ns). In particu-

lar, sampling occurs on the rising edge of the clock sig-

nal, requiring this edge to have the lowest possible

jitter. Any significant aperture jitter limits the SNR per-

formance of the ADC according to the following rela-

tionship:

Clock jitter is especially critical for high input frequency

applications. The clock input should always be consid-

ered as an analog input and routed away from any ana-

log or digital signal lines.

The MAX1421 clock input operates with a voltage

specifications for high and low periods, as stated in the

Electrical Characteristics.

S

1

2t

dB

IN

AJ

NR

=×

××









20

10

log

πƒ

MAX1421

REFIN

REFN

R

50

Ω

R

R

R

R

+1V

R

50

Ω

50

Ω

R

R

CML

1nF

0.22

µF

1nF

0.22

µF

1nF

0.22

µF

AGND

2

4

MAX4284

MAX4284

( )

REFP

+ 1V

(

)

2

+ 1V

(

)

2

2

4

2

Figure 3. Unbuffered External Reference Drive—Internal Reference Disabled