
MAX19708
10-Bit, 11Msps, Ultra-Low-Power
Analog Front-End
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29
Reference Configurations
The MAX19708 features an internal precision 1.024V
bandgap reference that is stable over the entire power-
supply and temperature ranges. The REFIN input pro-
vides two modes of reference operation. The voltage at
REFIN (VREFIN) sets the reference operation mode
(Table 18).
In internal reference mode, connect REFIN to VDD.
VREF is an internally generated 0.512V ±4%. COM,
REFP, and REFN are low-impedance outputs with
VCOM = VDD / 2, VREFP = VDD / 2 + VREF / 2, and
VREFN = VDD / 2 - VREF / 2. Bypass REFP, REFN, and
COM each with a 0.33F capacitor. Bypass REFIN to
GND with a 0.1F capacitor.
In buffered external reference mode, apply 1.024V
±10% at REFIN. In this mode, COM, REFP, and REFN
are low-impedance outputs with VCOM = VDD / 2,
VREFP = VDD / 2 + VREFIN / 4, and VREFN = VDD / 2 -
VREFIN / 4. Bypass REFP, REFN, and COM each with a
0.33F capacitor. Bypass REFIN to GND with a 0.1F
capacitor. In this mode, the Tx path full-scale output is
proportional to the external reference. For example, if
the VREFIN is increased by 10% (max), the Tx path full-
scale output is also increased by 10% or ±451mV.
Applications Information
Using Balun Transformer AC-Coupling
An RF transformer (Figure 9) 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 VDD / 2 DC level shift to the input. A 1:1
transformer can be used, or a step-up transformer can
be selected to reduce the drive requirements. In gener-
al, the MAX19708 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
(IAP, IAN, QAP, QAN) are balanced, and each of the
Rx ADC inputs only requires half the signal swing com-
pared to single-ended mode. Figure 10 shows an RF
transformer converting the MAX19708 Tx DAC differen-
tial analog outputs to single-ended.
Using Op-Amp Coupling
Drive the MAX19708 Rx ADC with op amps when a
balun transformer is not available. Figures 11 and 12
show the Rx ADC being driven by op amps for AC-cou-
pled single-ended and DC-coupled differential applica-
tions. Amplifiers such as the MAX4454 and MAX4354
provide high speed, high bandwidth, low noise, and
Table 18. Reference Modes
VREFIN
REFERENCE MODE
> 0.8V x VDD
Internal Reference Mode. VREF is internally generated to be 0.512V. Bypass REFP, REFN, and COM each
with a 0.33F capacitor.
1.024V ±10%
Buffered External Reference Mode. An external 1.024V ±10% reference voltage is applied to REFIN. VREF is
internally generated to be VREFIN / 2. Bypass REFP, REFN, and COM each with a 0.33F capacitor. Bypass
REFIN to GND with a 0.1F capacitor.
Figure 9. Balun Transformer-Coupled Single-Ended-to-
Differential Input Drive for Rx ADC
COM
IAP
IAN
25
Ω
0.1
μF
0.33
μF
25
Ω
0.1
μF
VIN
MAX19708
22pF
QAP
QAN
25
Ω
0.1
μF
0.33
μF
25
Ω
0.1
μF
VIN
22pF