THS4521 Datasheet, PDF (51/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

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THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
10 Power Supply Recommendations
The THS452x family 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 the
Application and Implementation. Split (or bipolar) supplies can be used with the THS452x family, 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
THS452x family 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 THS452x and provides a –230-
mV supply for the negative rail. Using the LM7705 provides an effective solution, as shown in the TI Designs
TIDU187, Extending Rail-to-Rail Output Range for Fully Differential Amplifiers to Include True Zero Volts.
11 Layout
11.1 Layout Guidelines
Figure 80 shows the THS4521EVM schematic. PCB layers 1 through 4 are shown in Figure 93; Table 9 lists the
bill of materials for the THS4521EVM as supplied from TI. It is recommended to follow the layout of the external
components near to the amplifier, ground plane construction, and power routing as closely as possible. Follow
these general guidelines:
• Signal routing should be direct and as short as possible into and out of the amplifier circuit.
• The feedback path should be short and direct.
• Ground or power planes should be removed from directly under the amplifier input and output pins.
• An output resistor is recommended in each output lead, placed as near to the output pins as possible.
• Two 0.1-μF power-supply decoupling capacitors should be placed as near to the power-supply pins as
possible.
• Two 10-μF power-supply decoupling capacitors should be placed within 1 inch of the device and can be
shared among multiple analog devices.
• A 0.22-μF capacitor should be placed between the V_OCM input pin and ground near to the pin. This capacitor
limits noise coupled into the pin.
• The PD pin uses TTL logic levels; a bypass capacitor is not necessary if actively driven, but can be used for
robustness in noisy environments whether driven or not.
• If input termination resistors R₁₀ and R₁₁ are used, a single point connection to ground on L2 is
recommended.
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