THS4531A_16 Datasheet, PDF (45/59 Pages) Texas Instruments – Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

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THS4531A
SLOS823C – DECEMBER 2012 – REVISED JANUARY 2016
These equations and design flow apply to any FDA. Using the feedback resistor value as a starting point is
particularly useful for current-feedback-based FDAs such as the LMH6554, where the value of these feedback
resistors determines the frequency response flatness. Similar tables can be built using the equations provided
here for other source impedances, R_F values, and gain ranges.
The TINA model correctly shows this actively-set input impedance in the single-ended to differential
configuration, and is a good tool to validate the gains, input impedances, response shapes, and noise issues.
9.2.7 Differential Input to Single-Ended Output Amplifier
Fully differential op amps like the THS4531A are not recommended for differential to single-ended conversion.
This application is best performed with an instrumentation amplifier or with a standard op amp configured as a
classic differential amplifier. See application section of the OPA835 data sheet (SLOS713).
10 Power Supply Recommendations
The THS4531A is principally intended to operate with a nominal single-supply voltage of 3 V to 5 V. Supply-
voltage tolerances are supported with the specified operating range of 2.5 V (10% low on a 3-V nominal supply)
and 5.5 V (8% high on a 5-V nominal supply). Supply decoupling is required, as described in Application and
Implementation. Split (or bipolar) supplies can be used with the THS4531A, as long as the total value across the
device remains less than 5.5 V (absolute maximum).
Using a negative supply to deliver a true swing to ground output in driving SAR ADCs may be desired. While the
THS4531A quotes a rail-to-rail output, linear operation requires approximately a 200-mV headroom to the supply
rails. One easy option for extending the linear output swing to ground is to provide the small negative supply
voltage required using the LM7705 fixed –230-mV, negative-supply generator. This low-cost, fixed negative-
supply generator accepts the 3- to 5-V positive supply input used by the THS4531A and provides a –230-mV
supply for the negative rail. Using the LM7705 provides an effective solution, as shown in the Extending Rail-to-
Rail Output Range for Fully Differential Amplifiers to Include True Zero Volts, TI Designs TIDU187.
11 Layout
11.1 Layout Guidelines
The THS4531A EVM (SLOU356) should be used as a reference when designing the circuit board. TI
recommends following the EVM layout of the external components near to the amplifier, ground plane
construction, and power routing as closely as possible. General guidelines are:
1. Signal routing should be direct and as short as possible into and out of the op amp.
2. The feedback path should be short and direct avoiding vias if possible.
3. Ground or power planes should be removed from directly under the amplifier’s input and output pins.
4. A series output resistor is recommended to be placed as near to the output pin as possible. See Figure 77
for recommended values given expected capacitive load of design.
5. A 2.2-µF power supply decoupling capacitor should be placed within 2 inches of the device and can be
shared with other op amps. For split supply, a capacitor is required for both supplies.
6. A 0.1-µF power supply decoupling capacitor should be placed as near to the power supply pins as possible.
Preferably within 0.1 inch. For split supply, a capacitor is required for both supplies.
7. The PD pin uses TTL logic levels referenced to the negative supply voltage (V_S-). When not used it should
tied to the positive supply to enable the amplifier. When used, it must be actively driven high or low and
should not be left in an indeterminate logic state. A bypass capacitor is not required, but can be used for
robustness in noisy environments.
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