THS1240 Datasheet, PDF (10/20 Pages) Texas Instruments – 12-BIT 40 MSPS IF SAMPLING COMMUNICATIONS ANALOG-TO-DIGITAL CONVERTER

THS1240
12-BIT 40 MSPS IF SAMPLING COMMUNICATIONS
ANALOG-TO-DIGITAL CONVERTER
SLAS279D – JUNE 2000 – REVISED JANUARY 2001
APPLICATION INFORMATION
using the analog input
The THS1240 obtains optimum performance when the analog signal inputs are driven differentially. The circuit
below shows the optimum configuration, see Figure 12. The signal is fed to the primary of an RF transformer.
Since the input signal must be biased around the common mode voltage of the internal circuitry, the common
mode (V_CM) reference from the THS1240 is connected to the center-tap of the secondary. To ensure a steady
low noise V_CM reference, the best performance is obtained when the V_CM output is connected to ground with
a 0.1-µF and 0.01-µF low inductance capacitor.
AC Signal Source
R0
Z0 = 50 Ω
1:1
50 Ω
T1-1T
VIN+
R
THS1240
50 Ω
VIN_–
VCM
0.01 µF
0.1 µF
Figure 12. Driving the THS1240 Analog Input With Impedance Matched Transmission Line
When it is necessary to buffer or apply a gain to the incoming analog signal, it is also possible to combine a
single-ended amplifier with an RF transformer as shown in Figure 13. For this application, a wide-band current
mode feedback amplifier such as the THS3001 is best. The noninverting input to the operational amplifier is
terminated with a resistor having an impedance equal to the characteristic impedance of the trace that sources
the IF input signal. The single-ended output allows the use of standard passive filters between the amplifier
output and the primary. In this case, the SFDR of the operational amplifier is not as critical as that of the A/D
converter. While harmonics generated from within the A/D converter fold back into the first Nyquist zone,
harmonics generated externally in the operational amplifier can be filtered out with passive filters.
1 kΩ
IF Input
RT
1 kΩ
_
10 Ω
+
THS3001
Impedance Ratio = 1:n
BPF
VIN+
THS1240
VIN_–
VCM
0.1 µF
0.01 µF
Figure 13. IF Input Buffered With THS3001 Operational Amplifier
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