Rainbow Electronics MAX108 - Manual (Page 17)

In a typical single-ended configuration, the analog
input signal (Figure 10a) enters the T/H amplifier stage
at the in-phase input (VIN+), while the inverted phase
input (VIN-) is reverse-terminated to GNDI with an
external 50
Ω
resistor. Single-ended operation allows for
an input amplitude of ±250mV. Table 3 shows a selec-
tion of input voltages and their corresponding output
codes for single-ended operation.
Differential Analog Inputs
To obtain a full-scale digital output with differential input
drive (Figure 10b), 250mVp-p must be applied between
VIN+ and VIN- (VIN+ = +125mV, and VIN- = -125mV).
Midscale digital output codes (01111111 or 10000000)
occur when there is no voltage difference between
VIN+ and VIN-. For a zero-scale digital output code, the
in-phase (VIN+) input must see -125mV and the invert-
ed input (VIN-) must see +125mV. A differential input
drive is recommended for best performance. Table 4
represents a selection of differential input voltages and
their corresponding output codes.
MAX108
±5V, 1.5Gsps, 8-Bit ADC with
On-Chip 2.2GHz Track/Hold Amplifier
______________________________________________________________________________________ 17
+2.8V
50 Ω
50 Ω
VIN+
ANALOG INPUTS ARE ESD PROTECTED
(NOT SHOWN IN THIS SIMPLIFIED DRAWING).
VIN-
GNDI
V EE
Figure 9. Simplified Analog Input Structure (Single-Ended/
Differential)
V IN+
V IN-
0V
+250mV
-250mV t
500mVp-p
FS ANALOG
INPUT RANGE
V
IN
= ±250mV
500mV
Figure 10a. Single-Ended Analog Input Signals
V IN+
V IN- +125mV
-125mV t
±250mV
FS ANALOG
INPUT RANGE
0V
250mV -250mV
Figure 10b. Differential Analog Input Signals
Table 3. Ideal Input Voltage and Output Code Results for Single-Ended Operation
0 -250mV 00000000 (zero scale) 0V
0 -250mV + 1LSB 0000001
0 0V
01111111
toggles
10000000
0V
0V
0 +250mV - 1LSB 11111111 0V
OUTPUT CODE VIN+ OVERRANGE BIT
1
VIN-
+250mV 11111111 (full scale) 0V