THS770012_15 Datasheet, PDF (14/35 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC Driver Amplifier

THS770012
SLOS669C – FEBRUARY 2010 – REVISED JANUARY 2012
TEST CIRCUITS
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OVERVIEW
The standard THS770012 evaluation module (EVM) is used for testing the typical performance shown in the
Typical Characteristics, with changes as noted below. The EVM schematic is shown in Figure 37. The signal
generators and analyzers used for most tests have single-ended 50Ω input and output impedance. The
THS770012 EVM is configured to convert to and from a single-ended 50Ω impedance by using RF transformers
or baluns 1:1 (CX2156NL from Pulse, supplied as a standard configuration of the EVM) to allow easy connection
to standard lab equipment. For line input termination, two 49.9Ω resistors (R5 and R6) are placed to ground on
the input transformer output pins (terminals 1 and 3). In combination with the 100Ω input impedance of the
device, the total impedance seen by the line is 50Ω.
Gain is set by placing external components R11, R12, R13, and R14 as described below in Setting the Gain
section.
A resistor network is used on the amplifier output to present various loads (R_L) and maintain line output
termination to 50Ω. Depending on the test conditions, component values are changed as shown in Table 1, or as
otherwise noted. As a result of the voltage divider on the output formed by the load component values, the
amplifier output is attenuated. The Loss column in Table 1 shows the attenuation expected from the resistor
divider. The output transformer causes slightly more loss, so these numbers are approximate.
Table 1. Load Component Values⁽¹⁾
LOAD R_L
100Ω
200Ω
400Ω
1kΩ
R15 AND R17
25Ω
86.6Ω
187Ω
487Ω
R16
Open
69.8Ω
57.6Ω
52.3Ω
LOSS
6dB
16.8dB
25.5dB
31.8dB
(1) The total load includes 50Ω termination by the test equipment. Components are chosen to achieve load and 50Ω line termination
through a 1:1 transformer.
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