THS3217 Datasheet, PDF (23/73 Pages) Texas Instruments – THS3217 DC to 800-MHz, Differential-to-Single-Ended, DAC Output Amplifier

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THS3217
SBOS766B – FEBRUARY 2016 – REVISED FEBRUARY 2016
8.2 Frequency Response Measurement
For D2S and full-signal path (D2S + OPS) characterization, the LMH3401, a very wideband, dc-coupled, single-
ended to differential amplifier was used. The LMH3401EVM was used as an interface between a single-ended
source and the differential input required by the D2S, shown in Figure 68. The LMH3401 provides an input
impedance of 50 Ω, and converts a single-ended input to a differential output driving through 50-Ω outputs on
each side to what is a 50-Ω termination at each input of the THS3217 D2S.
Network
Analyzer
GND Port 1
50
Port 2
50
LMH3401 EVM
+_
±⁺
THS3217 EVM
IN+ ₊
±
IN-
Figure 68. Frequency-Response Measurement: D2S and Full-Path (D2S + OPS) Circuit Configurations
The LMH3401 provides 7-GHz bandwidth with 0.1-dB flatness through 700 MHz. From the single-ended matched
input (using active match through an internal 12.5-Ω resistor), the LMH3401 produces a differential output with
16-dB gain to the internal output pins. Building out to a 50-Ω source by adding external 40.2-Ω resistors on both
differential outputs in series with the internal 10-Ω resistor, results in a net gain of 10 dB to the matched 50-Ω
load on the THS3217EVM.
The maximum output swing test for the D2S stage is 4 V_PP (see Figure 15 and Figure 16). With a fixed gain of 2
V/V, the tests in Figure 15 and Figure 16 require a 2-V_PP differential input. In order to achieve the 2-V_PP
differential swing at the D2S inputs, the LMH3401 internal outputs must drive a 4-V_PP differential signal around
the V_OCM of the LMH3401. This LMH3401 single-to-differential preamplifier is normally operated with ±2.5-V
supplies, and V_OCM set to ground. Under these conditions, the LMH3401 supports ±1.4 V on each internal output
pin; well beyond the maximum required for THS3217 D2S characterization of ±1 V.
The output of the LMH3401EVM connects directly to the Vin+ (J1) and Vin- (J2) SMA connectors on the
THS3217EVM, as shown in Figure 67. The physical spacing of the SMA connectors has been set to line up for a
direct (no cabling) connection between the two different EVMs using SMA barrels. For THS3217 designs that
must be evaluated before any DAC connection, consider using the LMH3401EVM as a gain of 10 dB, single-to-
differential interface to the inputs of the D2S stage. This setup allows single-ended sources to generate
differential output signals through the combined LMH3401EVM to THS3217EVM configuration. The D2S, small-
signal, frequency-response curves over input common-mode voltage (see Figure 13) were generated by
adjusting the LMH3401 voltage supplies and maintaining V_OCM at midsupply to preserve input headroom on the
LMH3401. In order to make single-ended, frequency-response measurements, the configuration shown in
Figure 69 was used.
Network
Analyzer
GND Port 1
50
Port 2
50
THS3217 EVM
+
±
Figure 69. Frequency-Response Measurement: OPS Inverting and Noninverting, Midscale Buffer, and
VREF Circuit Configurations
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