THS3215 Datasheet, PDF (54/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 2016
www.ti.com
In the example design of Figure 95, a 50-MHz, third-order Bessel filter is placed between the D2S output and the
external OPS input. Another 25-MHz, third-order Bessel filter is placed at the input of a very-high output-swing
THS3095 stage. A double-pole, double-throw (DPDT) relay selects the THS3095 path when the internal OPS
path is selected in the THS3215. Figure 95 shows this design. The key operational considerations in this design
include:
1. When the external input OPS path is selected, the 2-V_PP maximum D2S output swing experiences a 1.55-dB
insertion loss from the interstage filter between VO1 (pin 6) and VIN+ (pin 9). A standard value inductor is
used and the 464-Ω termination accounts for the internal 18.5-kΩ element. The 10-Ω resistor at VIN+
isolates the OPS input from the 52-pF filter capacitor. To recover the insertion loss and produce a maximum
5-V_PP output, the OPS gain is set to 2.96 V/V. When the interstage filter path is selected, the two DPDT
relays pass the OPS output on directly from the 49.9-Ω output matching resistor to V_O. Disable the THS3095
to conserve power.
2. To deliver 20 V_PP at the V_O output, select the THS3095 path. Select the internal OPS path to bypass the 50-
MHz filter (1.55-dB insertion loss) in order to give a maximum 5.9-V_PP output at VOUT (pin 11). The two
DPDT relays switch position, and the 49.9 Ω at the OPS output becomes part of the 25-MHz, third-order
Bessel filter into the THS3095 stage. This filter has a 1-dB insertion loss requiring a gain of 3.8 V/V in the
THS3095 to deliver 20 V_PP from the OPS output.
3. Figure 98 and Figure 99 show the frequency response and harmonic distortion performance of the dual
output-voltage system. The frequency response is normalized to 0 dB to make bandwidth comparisons
easier.
10.1.1.1.3 Application Curves
3
0
-3
-6
-9
-12
-15
1M
10M
Frequency (Hz)
5 V_PP
20 V_PP
100M 200M
D509
-40
-50
-60
-70
-80
-90
-100
1M
10M
Frequency (Hz)
HD2 -5 V_PP
HD3- 5 V_PP
HD2- 20 V_PP
HD3- 20 V_PP
D510
Figure 98. Frequency Response of the 5-V_PP and 20-V_PP
Channels
Figure 99. Harmonic Distortion Performance of the 5-V_PP
and 20-V_PP Channels
54
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