THS770012_15 Datasheet, PDF (19/35 Pages) Texas Instruments – Broadband, Fully-Differential, 14-/16-Bit ADC Driver Amplifier

THS770012
www.ti.com
SLOS669C – FEBRUARY 2010 – REVISED JANUARY 2012
Single-Ended to Differential
The THS770012 can be used to amplify and convert single-ended input signals to differential output signals. A
basic block diagram of the circuit with nominal gain of +12dB is shown in Figure 39. 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
200W
200W
Differential
Output
V_OUT+
V_OUT-
THS770012
Figure 39. Single-Ended Input to Differential Output Amplifier
Setting the Gain
Gain is adjustable by placing external components in positions R11, R12, R13, and R14 on the EVM. Table 2
below shows the component values for setting gain from +10dB to +13.7dB. The different configurations and
values change the effective value of the internal feedback, where the gain is determined by the resultant effective
R_F/R_G, where R_G = 50Ω
GAIN
+10dB
+11dB
+12dB
+13.7dB (AC gain), +12dB
(DC gain)⁽¹⁾
R11, R12
Open
Open
Open
Table 2. Gain Settings
R13, R14
140 Ω
523 Ω
Open
0.1 µF capacitors
Open
EFFECTIVE R_F
140 || 340 + 60 = 159 Ω
140 || 723 + 60 = 177 Ω
140 + 60 = 200 Ω
(140 || 200
+
60) ´ (140
200
+
200)
= 242 W
(1) Using 0.1 µF capacitors for R11 and R12 limits the low frequency response to the default value of +12dB at frequency below about
10kHz. For +13.7dB gain at DC, connect pins 13 and 18 of the device to a low impedance voltage source equal to V_OCM (+2.5V
nominal).
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 18 and Figure 34. 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 THS770012 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.
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