THS4521 datasheet(34/66 Pages) TI2

34 / 66 page

• Minimize the distance (< 0.1") from the power-supply pins to high-frequency, 0.1-μF decoupling capacitors.

At the device pins, the ground and power plane layout must not be in close proximity to the signal I/O

pins. Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling

capacitors. Always decouple the power-supply connections (on pins 4 and 7) with these capacitors. An

optional supply decoupling capacitor across the two power supplies (for bipolar operation) improves 2nd-

harmonic distortion performance. Use larger (2.2 μF to 6.8 μF) decoupling capacitors, effective at lower

frequency, on the main supply pins. Place these capacitors somewhat farther from the device. These

capacitors can be shared among several devices in the same area of the PCB.

• Although not always stated, tie the power disable pin to the positive supply when only an enabled channel is

desired.

• Virtually all ac characterization equipment expects a 50-Ω termination from the 50-Ω source, and a 50-Ω

single-ended source impedance from the device outputs to the 50-Ω sensing termination. This termination is

achieved in all characterizations (often with some insertion loss), but is not necessary for most applications.

Matching impedance is most often required when transmitting over longer distances. Tight layouts from

a source, through the THS4541, and on to an ADC input do not require doubly-terminated lines or filter

designs; the exception is if the source requires a defined termination impedance for correct operation (for

example, a SAW filter source).

• The amplifier signal path is flexible for single or split-supply operation. Most applications are intended to be

single supply, but any split-supply design can be used, as long as the total supply across the TH4541 is less

than 5.5 V and the required input, output, and common-mode pin headrooms to each supply are observed.

Left open, the Vocm pin defaults to near midsupply for any combination of split or single supplies used. The

disable pin is negative-rail referenced. Using a negative supply requires the disable pin to be pulled down to

within 0.7 V of the negative supply to disable the amplifier.

• External element values are normally assumed to be accurate and matched. In an FDA, match the feedback

resistor values and also match the (dc and ac) impedance from the summing junctions to the source on one

side and the reference or ground on the other side. Unbalancing these values introduces nonidealities in

the signal path. For the signal path, imbalanced resistor ratios on the two sides create a common-mode to

differential conversion. Also, mismatched Rf values and feedback ratios create some added differential output

error terms from any common-mode dc, ac signal, or noise terms. Approximating to standard 1% resistor

values is a typical approach and generally leads to some nominal feedback ratio mismatch. Mismatched

resistors or ratios do not in themselves degrade harmonic distortion. If there is meaningful CM noise or

distortion coming in, those errors are converted to a differential error through element or ratio mismatch.

THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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THS4521 Datasheet(PDF) 34 Page - Texas Instruments