THS3215 Datasheet, PDF (23/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

www.ti.com
THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 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 THS3215 D2S.
Network
Analyzer
GND Port 1
50
Port 2
50
LMH3401EVM
+_
±⁺
THS3215EVM
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 THS3215EVM.
The maximum output swing test for the D2S 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 THS3215 D2S characterization of ±1 V.
The output of the LMH3401EVM connects directly to the Vin+ (J1) and Vin– (J2) SMA connectors on the
THS3215EVM, as shown in Figure 67. The physical spacing of the SMA connectors lines up for a direct (no
cabling) connection between the two different EVMs using SMA barrels. For THS3215 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. This setup allows single-ended sources to generate differential output signals
through the combined LMH3401EVM to THS3215EVM 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
THS3215EVM
+
±
Figure 69. Frequency-Response Measurement: OPS Inverting and Noninverting, Midscale Buffer, and
VREF-Input Circuit Configurations
Copyright © 2016, Texas Instruments Incorporated
Product Folder Links: THS3215
Submit Documentation Feedback
23