MAX108
Offset Adjust
The MAX108 provides a control input (VOSADJ) to com-
pensate for system offsets. The offset adjust input is a
self-biased voltage divider from the internal +2.5V preci-
sion reference. The nominal open-circuit voltage is one-
half the reference voltage. With an input resistance of
typically 25kΩ, this pin may be driven by an external
10kΩ potentiometer (Figure 11) connected between
REFOUT and GNDI to correct for offset errors. This con-
trol provides a typical ±5.5LSB offset adjustment range.
Clock Operation
The MAX108 clock inputs are designed for either sin-
gle-ended or differential operation (Figure 12) with flexi-
ble input drive requirements. Each clock input is
terminated with an on-chip, laser-trimmed 50Ω resistor
to CLKCOM (clock-termination return). The CLKCOM
termination voltage can be connected anywhere
between ground and -2V for compatibility with standard
ECL drive levels.
The clock inputs are internally buffered with a preampli-
fier to ensure proper operation of the data converter,
even with small-amplitude sine-wave sources. The
MAX108 was designed for single-ended, low-phase-
noise sine-wave clock signals with as little as 100mV
amplitude (-10dBm). This eliminates the need for an
external ECL clock buffer and its added jitter.
Single-Ended Clock Inputs (Sine-Wave Drive)
Excellent performance is obtained by AC- or DC-cou-
pling a low-phase-noise sine-wave source into a single
clock input (Figure 13a, Table 5). For proper DC bal-
ance, the undriven clock input should be externally
50Ω reverse-terminated to GNDI.
The dynamic performance of the data converter is
essentially unaffected by clock-drive power levels from
-10dBm (100mV clock signal amplitude) to +10dBm
(1V clock signal amplitude). The MAX108 dynamic per-
±5V, 1.5Gsps, 8-Bit ADC with
On-Chip 2.2GHz Track/Hold Amplifier
18 ______________________________________________________________________________________
Table 4. Ideal Input Voltage and Output Code Results for Differential Operation
OUTPUT CODEVIN+
0-125mV 00000000 (zero scale)+125mV
0-125mV + 0.5LSB 00000001
00V
01111111
toggles
10000000
+125mV - 0.5LSB
0V
OVERRANGE BIT
0+125mV - 0.5LSB 11111111
1
VIN-
+125mV 11111111 (full scale)
-125mV + 0.5LSB
-125mV
SIMPLIFIED DRAWING).
(24 Seiten)
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