
Clock jitter is especially critical for undersampling
applications. Consider the clock input as an analog
input and route away from any analog input or other
digital signal lines. The MAX5864 clock input operates
with an OV
DD
/2 voltage threshold and accepts a 50%
±15% duty cycle.
Reference Configurations
The MAX5864 features an internal precision 1.024V
bandgap reference is stable over the entire power sup-
ply and temperature range. The REFIN input provides
two modes of reference operation. The voltage at REFIN
(V
REFIN
) sets reference operation mode (
Table 4
).
In internal reference mode, connect REFIN to V
DD
.
V
REF
is an internally generated 0.512V. COM, REFP,
and REFN are low-impedance outputs with V
COM
=
V
DD
/2, V
REFP
= V
DD
/2 + V
REF
/2, and V
REFN
= V
DD
/2 -
V
REF
/2. Bypass REFP, REFN, and COM each with a
0.33μF capacitor. Bypass REFIN to GND with a 0.1μF
capacitor.
In buffered external reference mode, apply 1.024V
±10% at REFIN. In this mode, COM, REFP, and REFN
are low-impedance outputs with V
COM
= V
DD
/2, V
REFP
= V
DD
/2 + V
REFIN
/4, and V
REFN
= V
DD
/2 - V
REFIN
/4.
Bypass REFP, REFN, and COM each with a 0.33μF
capacitor. Bypass REFIN to GND with a 0.1μF capaci-
tor. In this mode, the DAC
’
s full-scale output voltage
and common-mode voltage are proportional to the
external reference. For example, if the V
REFIN
is
increased by 10% (max), the DACs
’
full-scale output
voltage is also increased by 10% or
±
440mV, and the
common-mode voltage increases by 10%.
Applications Information
Using Balun Transformer AC-Coupling
An RF transformer (
Figure 7
) provides an excellent
solution to convert a single-ended signal source to a
fully differential signal for optimum ADC performance.
Connecting the center tap of the transformer to COM
provides a V
DD
/2 DC level shift to the input. A 1:1 trans-
former can be used, or a step-up transformer can be
selected to reduce the drive requirements. In general,
the MAX5864 provides better SFDR and THD with fully
differential input signals than single-ended signals,
especially for high-input frequencies. In differential
mode, even-order harmonics are lower as both inputs
(IA+, IA-, QA+, QA-) are balanced, and each of the
ADC inputs only requires half the signal swing com-
pared to single-ended mode.
Figure 8
shows an RF
transformer converting the MAX5864 DACs
’
differential
analog outputs to single ended.
M
Ultra-Low-Power, High Dynamic-
Performance, 22Msps Analog Front End
______________________________________________________________________________________
19
Table 4. Reference Modes
V
REFIN
REFERENCE MODE
>0.8 x V
DD
Internal reference mode. V
REF
is internally
generated to be 0.512V. Bypass REFP,
REFN, and COM each with a 0.33μF
capacitor.
1.024V ±10%
Buffered external reference mode. An
external 1.024V ±10% reference voltage
is applied to REFIN. V
REF
is internally
generated to be V
REFIN
/2. Bypass REFP,
REFN, and COM each with a 0.33μF
capacitor. Bypass REFIN to GND with a
0.1μF capacitor.
Figure 7. Balun-Transformer Coupled Single-Ended to
Differential Input Drive for ADCs
COM
IA+
IA-
25
0.1
μ
F
0.33
μ
F
25
0.1
μ
F
V
IN
MAX5864
22pF
22pF
QA+
QA-
25
0.1
μ
F
0.33
μ
F
25
0.1
μ
F
V
IN
22pF
22pF