ANALOG INPUT
The SPT7936 has a differential input that should have a
common mode voltage of +1.5 V. The input voltage range is
determined by the reference voltages which may be gener-
ated internally or applied externally.
The input of the SPT7936 can be configured in various ways
depending on if a single-ended or differential, AC- or DC-
coupled input is desired.
The AC coupled input is most conveniently implemented
using a transformer with a center tapped secondary winding.
The center tap is connected to the V
CM
pin as shown in
figure 2. To obtain low distortion, it is important that the
selected transformer does not exhibit core saturation at the
full-scale voltage. Excellent results are obtained with the Mini
Circuits T1-6T or T1-1T. Proper termination of the input is
important for input signal purity. A small capacitor across the
inputs attenuates kickback noise from the internal sample
and hold.
Figure 3 illustrates a solution (based on operational amplifi-
ers) that can be used if a a DC coupled single-ended input is
desired. The selection criteria of the buffer op-amps is as
follows:
- Open loop gain >75 dB
- Gain bandwidth product >50 MHz
- Total harmonic distortion
≤-75
dB
- Signal to Noise ratio >75 dB
REFERENCES
The SPT7936 can use either an internal or external voltage
reference. When the digital input EXTREF is high, the exter-
nal reference is used. When EXTREF is low, the internal
reference is used.
INTERNAL REFERENCE
The internal references are set at +1.0 V and +2.0 V. When
the internal reference is used, the full-scale range of the
analog input is set at
±1.0
V differential. Do not connect
external references when the internal reference is used.
EXTERNAL REFERENCE
When external references are used, the voltages applied to
the V
REF
+ and V
REF
- pins determine the input voltage range
which is equal to
±(V
REF
+ - V
REF
-). Externally generated
reference voltages must be connected to these pins and
should be symmetric about the common mode voltage. (See
figure 2, Typical Interface Circuit.)
COMMON MODE OUTPUT VOLTAGE
REFERENCE CIRCUIT
The SPT7936 has an on-board common mode voltage refer-
ence circuit (V
CM
). It is set at +1.5 V and can drive loads of up
to 20
µA.
This 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-differential con-
verter conversion circuit.
Figure 3 - DC-Coupled Single-Ended-to-Differential
Conversion (Power Supplies are Not
Shown)
R3
VCM
(R3)/2
_
+
Input
Voltage
(±0.5 V)
R2
+
15 pF
R2
51
Ω
R
R
+
_
R
VIN–
51
Ω
R3
R
_
51
Ω
VIN+
R
ADC
BIAS CIRCUITS
The best AC performance is achieved when the bias currents
are optimized for the selected sample rate. Two digital input
pins are provided to control the optimum internal bias cur-
rents. Table I shows the settings for Bias 0 and Bias 1 at
selected frequencies.
Table I - Frequencies for Biases 0 and 1
Typical Power
Dissipation
Bias 1 Bias 0 Description
Ext. Ref.
Int. Ref.
0
0
Sleep mode (power save)* 25 mW
36 mW
0
1
≤5
MHz sampling
61 mW
73 mW
1
0
≤20
MHz sampling
172 mW
184 mW
1
1
≤28
MHz sampling
248 mW
260 mW
*Clock = 28 MHz
POWER SUPPLIES AND GROUNDING
The SPT7936 is operated from a single power supply in the
range of 3.0 to 3.6 volts. Nominal operation is suggested to
be 3.3 volts. All power supply pins should be bypassed as
close to the package as possible. The analog and digital
grounds should be connected together with a ferrite bead as
shown in the typical interface circuit and as close to the ADC
as possible.
SPT7936
6
8/1/00
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
