25
TS8308500
2193A–BDC–04/03
Differential Versus
Single-ended Analog
Input Operation
The TS8308500 can operate at full speed in either the differential or single-ended
configuration.
This is explained by the fact the ADC uses a high input impedance differential preamplifier
stage, (preceeding the sample and hold stage), which has been designed in order to be
entered either in differential mode or single-ended mode.
This is true so long as the out-of-phase analog input pin V
INB is 50 terminated very closely to
one of the neighboring shield ground pins (52, 53, 58, 59) which constitute the local ground
reference for the in-phase analog input pin (V
IN).
Thus the differential analog input preamplifier will fully reject the local ground noise (and any
capacitively and inductively coupled noise) as common mode effects.
In a typical single-ended configuration, enter on the (V
IN) input pin, with the inverted phase
input pin (V
INB) grounded through the 50 termination resistor.
In a single-ended input configuration, the in-phase input amplitude is 0.5V, centered on 0V (or
-2 dBm into 50
). The inverted phase input is at ground potential through the 50 termination
resistor.
However, dynamic performances can be somewhat improved by entering either analog or
clock inputs in differential mode.
Typical Single-ended
Analog Input
Configuration
Figure 29. Typical Single-ended Analog Input Configuration
Clock Inputs (CLK,
CLKB)
The TS8308500 can be clocked at full speed without noticeable performance degradation in
either the differential or single-ended configuration.
This is explained by the fact the ADC uses a differential preamplifier stage for the clock buffer,
which has been designed in order to be entered either in differential or single-ended mode.
The recommended sinewave generator characteristics are typically -120 dBc/Hz phase noise
floor spectral density, at 1 kHz from carrier, assuming a single tone 4 dBm input for the clock
signal.
Single-ended Clock
Input (Ground
Common Mode)
Although the clock inputs were intended to be driven differentially with nominal -0.8V/-1.8V
ECL levels, the TS8308500 clock buffer can manage a single-ended sinewave clock signal
centered around 0V. This is the most convenient clock input configuration as it does not
require the use of a power splitter.
50
(on package)
1 M
3 pF
-250
250
500 mV
t
[mV]
VIN
VIN =
±250 mV = 500 mV diff
VIN or VINB double pad (pins 54, 55 or 56, 57)
VIN or VINB
50
reverse termination
500 mV
Full Scale
analog input
VINB = 0V
VINB