SPT7755 Ver la hoja de datos (PDF) - Cadeka Microcircuits LLC.
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SPT7755 Datasheet PDF : 8 Pages
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TYPICAL INTERFACE CIRCUIT
VRBF, VRBS, VRTF, VRTS, VRM (REFERENCE INPUTS)
The circuit in figure 1 is intended to show the most elabo-
rate method of achieving the least error by correcting for
integral linearity, input induced distortion, and power sup-
ply/ground noise. This is achieved by the use of external
reference ladder tap connections, input buffer, and supply
decoupling. Please contact the factory for the SPT7755
evaluation board application note that contains more
details on interfacing the SPT7755. The function of each
pin and external connections to other components is as
follows:
VEE, AGND, DGND
VEE is the supply pin with AGND as ground for the device.
The power supply pins should be bypassed as close to the
device as possible with at least a .01 µF ceramic capaci-
tor. A 10 µF tantalum can also be used for low frequency
suppression. DGND is the ground for the ECL outputs and
is to be referenced to the output pulldown voltage and ap-
propriately bypassed as shown in figure 1.
VIN (ANALOG INPUT)
There are two analog input pins that are tied to the same
point internally. Either one may be used as an analog input
sense and the other for input force. This is convenient for
testing the source signal to see if there is sufficient drive
capability. The pins can also be tied together and driven by
the same source. The SPT7755 is superior to similar de-
vices due to a preamplifier stage before the comparators.
This makes the device easier to drive because it has con-
stant capacitance and induces less slew rate distortion.
There are two reference inputs and one external reference
voltage tap. These are –2 V (VRB force and sense), mid-
tap (VRM) and AGND (VRT force and sense). The refer-
ence pins and tap can be driven by op amps as shown in
figure 1 or VRM may be bypassed for limited temperature
operation. These voltage inputs can be bypassed to
AGND for further noise suppression if so desired.
Table I – Output Coding
VIN
D8
D7 . . . D8
>–0.5 LSB
1
10000000
–0.5 LSB
1
10000000
0
10000000
–1.5 LSB
0
10000000
0
10000001
•
•
•
•
•
•
•
•
•
–1.0 V
0
11000000
0
01000000
•
•
•
•
•
•
•
•
•
–2.0 V +0.5 LSB
0
00000001
0
00000000
<(–2.0 V +0.5 LSB)
0
00000000
Indicates the transition between the two codes
THERMAL MANAGEMENT
The typical thermal impedance is as follows:
CLK, CLK (CLOCK INPUTS)
ΘCA = +17 °C/W in still air with no heat sink
The clock inputs are designed to be driven differentially
with ECL levels. The duty cycle of the clock should be kept
at 50% to avoid causing larger second harmonics. If this is
not important to the intended application, then duty cycles
other than 50% may be used.
D0 TO D8, DR, DR, (A AND B)
The digital outputs can drive 50 Ω to ECL levels when
pulled down to –2 V. When pulled down to –5.2 V, the out-
puts can drive 130 Ω to 1 kΩ loads. All digital outputs are
grey code with the coding as shown in table I. CADEKA rec-
ommends using differential receivers on the outputs of the
data ready lines to ensure the proper output rise and fall
times.
We highly recommend that a heat sink be used for this
device with adequate air flow to ensure rated performance
of the device. We have found that a Thermalloy 17846
heat sink with a minimum air flow of 1 meter/second (200
linear feet per minute) provides adequate thermal perfor-
mance under laboratory tests. Application specific condi-
tions should be taken into account to ensure that the
device is properly heat sinked.
SPT7755
5
October 2002
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