12-Bit, 40Msps, +3.3V, Low-Power ADC
with Internal Reference
AVDD
R
R
AVDD
2
R
AVDD
4
R
50Ω
0.22µF
MAX4284
AVDD
2
50Ω
R 0.22µF
MAX4284
AVDD
4
R
50Ω
0.22µF
R
R
AGND
+1V
Figure 3. Unbuffered External Reference Drive—Internal Reference Disabled
( ) CML
AVDD
2
1nF
( ) REFP
AVDD + 1V
2
1nF
MAX1421
( ) REFN AVDD + 1V
2
1nF
REFIN
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, CLK)
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:
SNRdB
=
20
×
log10
2π
×
1
ƒIN
×
tAJ
where fIN represents the analog input frequency and
tAJ is the aperture jitter.
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
threshold set to AVDD / 2. Clock inputs must meet the
specifications for high and low periods, as stated in the
Electrical Characteristics.
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