THS770006 Datasheet, PDF (19/39 Pages) National Semiconductor (TI) – Broadband, Fully-Differential, 14-/16-Bit ADC DRIVER AMPLIFIER

THS770006
www.ti.com
SBOS520B – JULY 2010 – REVISED JANUARY 2012
Single-Ended to Differential
The THS770006 can be used to amplify and convert single-ended input signals to differential output signals. A
basic block diagram of the circuit is shown in Figure 35. The gain from the single-ended input to the differential
output is 6dB. In order to maintain proper balance in the amplifier and avoid offsets at the output, the alternate
input must be biased and the impedance matched to the signal input. For example, if a 50Ω source biased to
2.5V provides the input, the alternate input should be tied to 2.5V through 50Ω. If a 50Ω source is ac-coupled to
the input, the alternate input should be ac-coupled to ground through 50Ω. Note that the ac coupling should
provide a similar frequency response to balance the gain over frequency.
V_REF
Bias and
Impedance
Match
50W
Single-Ended
Input
50W
100W
100W
Differential
Output
V_OUT+
V_OUT-
THS770006
Figure 35. Single-Ended Input to Differential Output Amplifier
Setting the Output Common-Mode Voltage
The V_OCM input controls the output common-mode voltage. V_OCM has no internal biasing network and must be
driven by an external source or resistor divider network to the positive power supply. In ac-coupled applications,
the V_OCM input impedance and bias current are not critical, but in dc-coupled applications where more accuracy
is desired, the input bias current of the pin should be considered. For best harmonic distortion with V_OUT = 3V_PP,
the V_OCM input should be maintained within the operating range of 2.25V to 2.75V. The V_OCM input voltage can
be operated outside this range if lower output swing is used or distortion degradation is allowed, and increased
bias current into the pin is acceptable. For more information, see Figure 12 and Figure 31. It is recommended to
use a 0.1µF decoupling capacitor from the V_OCM pin to ground to prevent noise and other spurious signals from
coupling into the common-mode loop of the amplifier.
Input Common-Mode Voltage Range
The THS770006 is designed primarily for ac-coupled operation. With input dc blocking, the input common-mode
voltage of the device is driven to the same voltage as V_OCM by the outputs. Therefore, as long as the V_OCM input
is maintained within the operating range of 2.25V to 2.75V, the input common-mode of the main amplifier is also
maintained within its linear operating range of 2.25V to 2.75V. If the device is used with dc coupled input, the
driving source needs to bias the input to its linear operating range of 2.25V to 2.75V for proper operation.
Operation with Split Supply ±2.5V
The THS770006 can be operated using a split ±2.5V supply. In this case, V_S+ is connected to +2.5V, and GND
(and any other pin noted to be connected to GND) is connected to –2.5V. As with any device, the THS770006 is
impervious to what the user decides to name the levels in the system. In essence, it is simply a level shift of the
power pins by –2.5V. If everything else is level-shifted by the same amount, the device sees no difference. With
a ±2.5V power supply, the V_OCM range is 0V ±0.25V; therefore, power-down levels are –2.5V = on and +2.5V =
off, and input and output voltage ranges are symmetrical about 0V. This design has certain advantages in
systems where signals are referenced to ground, and as noted in the following section, for driving ADCs with low
input common-mode voltage requirements in dc-coupled applications.
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