THS4551 Datasheet, PDF (49/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
Typical Applications (continued)
10.2.2 Differential Transimpedance Output to a High-Grade Audio PCM DAC Application
The highest-grade audio digital-to-analog converters (DACs) are a differential current-mode output. These
devices normally suggest a two-amplifier transimpedance stage to hold the DAC output voltages fixed when the
amplifiers produce a differential voltage swing at the outputs. Often, the differential voltage swing is then
converted to single-ended in a differencing amplifier stage to drive headphone loads (see Figure 35 in the
OPA1611). The emerging high-power class D audio amplifiers often require differential inputs. Applying the
THS4551 as a differential transimpedance stage offers a simple solution for very low-distortion, differential-output
audio channels.
Starting with the output specifications for a very high-performance PCM1792A audio DAC, the requirements for
the THS4551 interface can be extracted. The DAC is a current-sourcing device that requires the outputs to be
held at ground when using a transimpedance amplifier. Using the DAC 3.3-V supply and the LM27762 low-noise,
low-dropout (LDO) regulator and inverter provides a ±2.5-V supply to the THS4551. Operating the THS4551 on
±2.5-V supplies places all nodes in range for an input V_CM equal to GND (and the DAC output voltages as well)
and an FDA output V_OCM equal to GND.
The center current in Table 5 is a fixed 6.2-mA dc current coming out of the DAC outputs regardless of the DAC
code. This dc common-mode current can be absorbed by the –2.5-V supply at the input pins to hold the DAC
compliance voltage and FDA input pins at ground. The FDA controls the output common-mode voltage, set to
ground in this case, whereas the input pin voltage (which does not move with the DAC output differential current)
is controlled with a resistor to the negative supply.
ANALOG OUTPUT
Gain error
Gain mismatch, channel-to-channel
Bipolar zero (BPZ) error
Output current
Center current
Table 5. PCM1792A Analog Output Specification
TEST CONDITION
MIN
–6
–3
At BPZ
–2
Full-scale (0 dB)
At BPZ
TYP MAX
±2
6
±0.5
3
±0.5
2
7.8
–6.2
UNIT
% of FSR
% of FSR
% of FSR
mA_PP
mA
This bias is provided by the 402-Ω resistors to –2.5 V, as illustrated in Figure 90. This design takes the
differential 7.8 mA_PP from the DAC and produces a ±1.46-V swing on each output of the THS4551. This
configuration gives a full-scale differential 5.85 V_PP available on the ±2.5-V supply design centered at ground at
both the inputs and outputs. Although the LM27762 provides a very-low noise, –2.5-V supply, using 0.1%
resistors in the current sink path to the –2.5-V supply as well as the feedback resistors limits any common-mode
noise on the –2.5-V supply to differential mode conversion at the FDA outputs.
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