GENERAL DESCRIPTION
The SPT7760 is one of the fastest monolithic 8-bit parallel
flash A/D converters available today. The nominal conver-
sion rate is 1 GSPS and the analog bandwidth is in
excess of 900 MHz. A major advance over previous flash
converters is the inclusion of 256 input preamplifiers
between the reference ladder and input comparators (see
block diagram). This not only reduces clock transient kick-
back to the input and reference ladder due to a low AC
beta, but also reduces the effect of the dynamic state of
the input signal on the latching characteristics of the input
comparators. The preamplifiers act as buffers and stabi-
lize the input capacitance so that it remains constant over
different input voltage and frequency ranges and therefore
Figure 1 – SPT7760 Typical Interface Circuit
DRB
V
IN
**
50
W
V
IN
V
IN
DRB
DRA
makes the part easier to drive than previous flash convert-
ers. The preamplifiers also add a gain of two to the input
signal so that each comparator has a wider overdrive or
threshold range to “trip” into or out of the active state. This
gain reduces metastable states that can cause errors at
the output.
The SPT7760 has true differential analog and digital data
paths from the preamplifiers to the output buffers (Current
Mode Logic) for reducing potential missing codes while
rejecting common mode noise.
Signature errors are also reduced by careful layout of the
analog circuitry. The output drive capability of the device
can provide full ECL swings into 50
Ω
loads.
U3
DRB (DATA READY)
DRB (DATA READY)
DRA (DATA READY)
50
W
50
W
DRA
U3
DRA (DATA READY)
2.0 V Pulldown (Digital)
V
RTF
R
22
W
V
RTS
D8B (OVR)
D7B (MSB)
D6B
D5B
D4B
D3B
D2B
D1B
D0B (LSB)
D8A (OVR)
D7A (MSB)
D6A
D5A
D4A
D3A
D2A
D1A
D0A (LSB)
+
U1
R
*
V
RM
Convert
U2
CLK
CLK
50
W
50
W
AGND
DGND
50
W
50
W
V
EE
+
2.0 V
Reference
22
W
2N2907
U1
V
RBS
V
RBF
*
5.2 V
5.2 V
.1 µF
FB = Ferrite bead
2.0 V
Pulldown
(Digital)
2 V
Pulldown
(Analog)
U1 = OP291 or equivalent with low offset/noise.
R = 1 kW; 0.1% matched.
= AGND
= DGND
U2 = ON Semiconductor ECLinPS LITE, MC10EL16,
differential receiver with 250 ps (typ) propagation delay.
U3 = MC10EL16 or MC100EL16.
*
= 10 µF Tantalum Capacitor and 0.1 µF Chip Capacitor
*
5.2 V
FB
** = Care must be taken to avoid exceeding the maximum rating
for the input, especially during power up sequencing of the
analog input driver.
4
SPT7760
October 2002
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
