Rainbow Electronics MAX1420 - Manual (Page 14)

MAX1420
with high-speed op amps. Select the R ISO and C IN val-
ues to optimize the filter performance, to suit a particu-
lar application. For the application in Figure 7, an
isolation resistor (R
ISO ) of 50 Ω is placed before the
capacitive load to prevent ringing and oscillation. The
22pF C
IN capacitor acts as a small bypassing capacitor.
Connecting C IN from INN to INP may further improve
dynamic performance.
Using Transformer Coupling
An RF transformer (Figure 8) provides an excellent
solution to convert a single-ended signal to a fully dif-
ferential signal, required by the MAX1420 for optimum
performance. Connecting the center tap of the trans-
former to CML provides an AV
DD /2 DC level shift to 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 MAX1420 provides better SFDR and THD
with fully differential input signals over single-ended
input signals, especially for very high input frequencies.
In differential input mode, even-order harmonics are sup-
pressed and each input 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 MAX1420 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
1nF to AGND. AV
DD should be bypassed with a similar
network of a 10µF bipolar capacitor in parallel with two
ceramic capacitors of 1nF and 0.1µF. Follow the same
rules to bypass the digital supply DV
DD to DGND.
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 ground (DGND) on the ADCs
package. Join the two ground planes at a single point,
such that the noisy digital ground currents do not inter-
fere 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, dig-
ital systems ground plane (e.g., downstream output
buffer or DSP ground plane). Route high-speed digital
signal traces away from sensitive analog traces and
remove digital ground and power planes from under-
neath digital outputs. Keep all signal lines short and
free of 90 degree turns.
12-Bit, 60Msps, +3.3V, Low-Power ADC
with Internal Reference
MAX1420
T1
N.C.
V IN
6 1
5 2
4 3
22pF
22pF
1nF
0.1 µF
0.22 µF
25 Ω
25 Ω
MINICIRCUITS
T1–1T–KK81
INN
INP
CML
44pF
*
*
*
*REPLACE BOTH 22pF CAPS WITH 44pF BETWEEN
INP AND INN TO IMPROVE DYNAMIC PERFORMANCE.
Figure 8. Using a Transformer for AC-Coupling
14 _____________________________________________________________________________________