THS770012 Datasheet, PDF (18/35 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC Driver Amplifier

THS770012
SLOS669C – FEBRUARY 2010 – REVISED JANUARY 2012
THEORY OF OPERATION
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GENERAL DESCRIPTION
The THS770012 is a wideband, fully differential amplifier designed and optimized specifically for driving 14-bit
and 16-bit ADCs at input frequencies up to 200MHz. This device provides high bandwidth, low distortion, and low
noise, which are critical parameters in high-speed data acquisition systems that require very high dynamic range,
such as wireless base stations and test and measurement applications. It also makes an excellent differential
amplifier for general-purpose, high-speed differential signal chain and short line-driver applications. The device
has an operating power-supply range of 4.75V to 5.5V. The THS770012 has proprietary circuitry to provide very
fast recovery from overdrive conditions and has a power-down mode for power saving. The THS770012 is
offered in a Pb-free (RoHS compliant) and green, QFN-24 thermally-enhanced package. It is characterized for
operation over the industrial temperature range of –40°C to +85°C.
The amplifier uses two negative-feedback loops. One is for the primary differential amplifier and the other
controls the common-mode operation.
Primary Differential Amplifier
The primary amplifier of the THS770012 is a fully-differential op amp with on-chip gain setting resistors. The gain
of the device can be changed by using external components as described in Setting the Gain section below. The
nominal gain with no external connections is set to +12dB with R_F = 200Ω and R_G = 50Ω.
V_OCM Control Loop
The output common-mode voltage is controlled through a second negative-feedback loop. The output
common-mode voltage is internally sensed and compared to the V_OCM pin. The loop then works to drive the
difference, or error voltage, to zero in order to maintain the output common-mode voltage = V_OCM (within the loop
gain and bandwidth of the loop). For more details on fully-differential amplifier theory and use, see application
report SLOA054, Fully-Differential Amplifiers, available for download from www.ti.com.
OPERATION
Differential to Differential
The THS770012 is a fully-differential amplifier that can be used to amplify differential input signals to differential
output signals. A basic block diagram of the circuit with nominal gain of +12dB is shown in Figure 38. The
differential input to differential output configuration gives the best performance; the signal source and load should
be balanced.
Differential
Input
V_IN-
V_IN+
50W
50W
200W
200W
Differential
Output
V_OUT+
V_OUT-
THS770012
Figure 38. Differential Input to Differential Output Amplifier
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