Rainbow Electronics MAX1211 - Manual (Page 19)

MAX1211
65Msps, 12-Bit, IF Sampling ADC
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power-down. The reference circuit requires 10ms to
power up and settle when power is applied to the
MAX1211 or when PD transitions from high to low.
The internal bandgap reference and buffer generate
REFOUT to be 2.048V with a +100ppm/°C temperature
coefficient. Connect an external
≥0.1µF bypass capacitor
from REFOUT to GND for stability. REFOUT sources up to
1.4mA and sinks up to 100µA for external circuits with a
load regulation of 35mV/mA. Short-circuit protection limits
I
REFOUT
to a 2.1mA source current when shorted to GND
and a 240µA sink current when shorted to V DD .
Analog Inputs and Reference
Configurations
The MAX1211 full-scale analog input range is ±V REF with
a common-mode input range of V
DD
/ 2 ±0.8V. V
REF
is
the difference between V REFP and V REFN . The MAX1211
provides three modes of reference operation. The volt-
age at REFIN (V
REFIN ) sets the reference operation
mode (Table 1).
To operate the MAX1211 with the internal reference, con-
nect REFOUT to REFIN either with a direct short or
through a resistive divider. 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. The REFIN input impedance is very large
(>50M Ω). When driving REFIN through a resistive divider,
use resistances ≥10kΩ to avoid loading REFOUT.
Buffered external reference mode is virtually identical to
internal reference mode except that the reference source
is derived from an external reference and not the
MAX1211 REFOUT. In buffered external reference mode,
apply a stable 0.7V to 2.3V source at REFIN. In this
mode, COM, REFP, and REFN are low-impedance out-
puts with V
COM = V
DD
/ 2, V REFP = V
DD
/ 2 + V REFIN / 4,
and V REFN = V
DD
/ 2 - V REFIN / 4.
To operate the MAX1211 in the unbuffered external refer-
ence mode, connect REFIN to GND. Connecting REFIN
to GND deactivates the on-chip reference buffers for
COM, REFP, and REFN. With their buffers deactivated,
COM, REFP, and REFN become high-impedance inputs
and must be driven through separate, external reference
sources. Drive V
COM to V DD / 2 ±5%, and drive REFP
and REFN such that V COM = (V REFP + V REFN ) / 2. The
analog input range is ±(V REFP - V REFN ).
All three modes of reference operation require the same
bypass capacitor combination. Bypass COM with a 0.1µF
capacitor in parallel with a
≥2.2µF capacitor to GND.
Bypass REFP and REFN each with a 0.1µF capacitor to
GND. Bypass REFP to REFN with a 1µF capacitor in par-
allel with a 10µF capacitor. Place the 1µF capacitor as
close to the device as possible. Bypass REFIN and
REFOUT to GND with a 0.1µF capacitor.
For detailed circuit suggestions, see Figures 12 and
Figures 13.
Clock Input and Clock Control Lines
(CLKP, CLKN, CLKTYP)
The MAX1211 accepts both differential and single-
ended clock inputs. For single-ended clock input oper-
ation, connect CLKTYP to GND, CLKN to GND, and
drive CLKP with the external single-ended clock signal.
For differential clock input operation, connect CLKTYP
to OVDD or VDD and drive CLKP and CLKN with the
external differential clock signal. To reduce clock jitter,
the external single-ended clock must have sharp falling
edges. Consider the clock input as an analog input and
route it away from any other analog inputs and digital
signal lines.
CLKP and CLKN are high impedance when the
MAX1211 is powered down (Figure 4).
V REFIN REFERENCE MODE
35% V REFOUT to 100% V REFOUT
In t e r n a l re f e r e n c e m o d e . RE FIN i s d r i ven b y RE FOU T ei ther thr oug h a d i r ect shor t or a r esi sti ve
d i vi d er . V C OM = V D D / 2, V RE F P = V D D / 2 + V RE F IN / 4, and V RE F N = V D D / 2 - V RE F IN / 4.
0.7V to 2.3V
Buffered external reference mode . An external 0.7V to 2.3V reference voltage is applied
to REFIN. V COM = V DD / 2, V REFP = V DD / 2 + V REFIN / 4, and V REFN = V DD / 2 - V REFIN / 4.
<0.5V
Unbuffered external reference mode . REFP, REFN, and COM are driven by external
reference sources. V REF is the difference between the externally applied V REFP and V REFN .
Table 1. Reference Modes