REFERENCES
To save on parts count, design time, and PC board real
estate, the SPT7720 utilizes an internal reference. No
other external components are required to implement this
feature.
VOLTAGE REFERENCE CIRCUIT
The SPT7720 has an on-board voltage reference circuit
(V
REF
). It is 2.5 volts and is capable of driving 50 µA loads
typically. The circuit is commonly used to drive the center
tap of the RF transformer in fully differential applications.
For single-ended applications, this output can be used to
provide the level shifting required for the single-to-differen-
tial converter conversion circuit.
ENCODE INPUT
The ENCODE input on the SPT7720 can be driven by ei-
ther a single-ended or differential clock circuit and can
handle TTL, PECL, and CMOS signals. When operating at
high sample rates it is important to keep the pulse width of
the duty signal as close to 50% as possible. For TTL/
CMOS single-ended ENCODE inputs, the rise time of the
signal also becomes an important consideration. The
ENCODE input is 300
Ω
into a bipolar differential pair.
ENCODE
is internally biased to 1.5 V with a Thevenin
equivalent of 5.25 kΩ.
DIGITAL INPUTS
The DS input is 35
Ω
into one side of a differential pair.
There is a two-diode clamp from DS to
DS
in both direc-
tions.
DS
is biased to 1.5 V with a Thevenin equivalent of
5.25 kΩ.
The
DEMUX
pin is input to one side of a CMOS differential
pair. The other side is internally biased to 1.5 V and does
not connect to the outside.
DIGITAL OUTPUTS
The output circuitry of the SPT7720 has been designed to
be able to support two separate output modes. The
demuxed (double-wide) mode supports interleaved data
output. The single-channel mode is not demuxed and can
support direct output at speeds up to 100 MSPS.
The output format is straight binary (table I).
Table I – Output Data Format
Output Code
D7–D0
+FS
1111 1111
+FS – 1/2 LSB
1111 111Ø
+1/2 FS
ØØØØ ØØØØ
–FS + 1/2 LSB
0000 000Ø
–FS
0000 0000
Ø indicates the flickering bit between logic 0 and 1
Analog Input
The data output mode is set using the
DEMUX
input (pin
42). Table II describes the mode switching options.
Table II – Output Data Modes
Output Mode
Interleaved Dual Channel Output
Single Channel Data Output
(Bank A only 100 MSPS max)
DEMUX
0
1
EVALUATION BOARD
The EB7720 evaluation board is available to aid designers
in demonstrating the full performance of the SPT7720.
This board includes a clock driver and reset circuit, adjust-
able references and common mode, a single-ended to dif-
ferential input buffer and a single-ended to differential
transformer (1:1). An application note (AN7720) describ-
ing the operation of this board, as well as information on
the testing of the SPT7720, is also available. Contact the
factory for price and availability of the EB7720.
SPT7720
8
5/9/01
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
