SPT7851SIT データシートの表示（PDF） - Signal Processing Technologies
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SPT7851SIT Datasheet PDF : 8 Pages
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ANALOG INPUT
The input of the SPT7851 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 wind-
ing. The center tap is connected to the VCM pin as shown in
figure 2. To obtain low distortion, it is important that the se-
lected transformer does not exhibit core saturation at the
full-scale voltage. Proper termination of the input is impor-
tant for input signal purity. A small capacitor across the in-
puts 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
Figure 3 – DC-Coupled Single-Ended to Differential
Conversion (power supplies and bypassing
are not shown)
R3
VCM
R3
–
+
Input
Voltage
(±0.5 V)
(R3)/2
R
R2
R2
R
–
51 Ω
+
15 pF
ADC
VIN+
VIN–
51 Ω
+
R
–
R
R
51 Ω
POWER SUPPLIES AND GROUNDING
The SPT7851 is operated from a single power supply in the
range of 2.8 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.
REFERENCES
The SPT7851 has a differential analog input. The voltages
applied to the VREF+ and VREF– pins determine the input
voltage range and are equal to ±(VREF+ – VREF–). This volt-
age range will be symmetrical about the common mode volt-
age. Externally generated reference voltages must be con-
nected to these pins. (See figure 2, Typical Interface
Circuit.) For best performance, these voltages should be
symmetrical about the midpoint of the supply voltage.
COMMON MODE VOLTAGE
REFERENCE CIRCUIT
The SPT7851 has an on-board common mode voltage ref-
erence circuit (VCM). It is typically one-half of the supply volt-
age and can drive loads of up to 20 µA. This circuit is com-
monly 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 converter conversion circuit.
BIAS CURRENT CIRCUITS
The bias currents suggested (Bias 1 and Bias 2 in figure 2)
optimize device performance for the stated sample rate of
20 MSPS. To achieve the best dynamic performance when
operating the device at sample rates other than 20 MSPS,
the bias current levels should be adjusted. Table I shows the
settings for Bias 1 and Bias 2 for selected sample rates.
The “Bias Voltage vs Bias Current” graphs on page 4 show
the relationship between the bias current and the bias
voltage. Please refer to the application note for more
information.
Table I – Sample Rate Settings
Sample Rate (MHz) Bias 1 (µA)
1
30
5
50
10
70
20
90
Bias 2 (µA)
3.0
6.0
7.5
9.5
SPT
6
SPT7851
8/1/00
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