ADA4356ABCZ datasheet(38/61 Pages) AD

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Data Sheet

ADA4356

analog.com

Rev. 0

38 of 61

Figure 84. Example Implementation of External High-Speed Current Divider Circuit

The following subsections explain factors to consider when selecting current divider components.

TIAREF Buffering Requirement

Ball TIAREF (F1) is connected to the ADA4356’s internal 1.65V reference via a series 2kΩ resistor for protection. Any

current drawn from TIAREF causes an error in the DC bias voltage, due to an IR drop across the 2kΩ resistor. Instead

of drawing current directly from TIAREF, it is recommended to buffer TIAREF’s output with a voltage buffer A1. This

buffer can rapidly charge C1 with high currents without introducing errors in the DC bias voltage.

Buffer Selection

Buffer A1 must have high gain bandwidth, fast slew rate, low output impedance over the input signal frequency

range, and good stability while driving large capacitors in the range of 10’s of µFs. The LTC6268/LTC6269 is an

excellent choice for this buffer.

Resistor Selection

The value of resistor R2 must not be too high, since current I2 flowing through R2 causes an IR drop that modulates

the bias voltage of the photodiode. With a maximum I2 of 300µA, selecting values of R2 below 1kΩ keep this bias

error below 300mV.

Capacitor Selection

It is recommended to place multiple smaller capacitors in parallel to create C1, to cover a wider frequency range

while handling relatively high currents.

A real capacitor’s bandwidth is limited to its self-resonant frequency. All real capacitors have unavoidable parasitic

package inductance. When combined with the normal capacitor value, this results in an LC tank. Above the self-

resonant frequency of the LC tank, the nominal capacitor acts as an inductor.

Larger capacitor packages can handle higher currents, but have higher parasitic inductance, which lowers the self-

resonant frequency of the LC tank and thus the effective frequency of the capacitor.

The self-resonant LC tank may cause instability problems at the output of buffer A1. To reduce the risk of

oscillations, it is recommended to place a small series resistor (0.1Ω) between the capacitor and ground to reduce

the quality factor (Q) of the self-resonant tank.

Recommended values for a combination C1 are 22µF, 100nF, and 10nF in parallel.

Switch Selection

Selecting a switch S1 requires considering how switch nonidealities affect the current divider circuit.

INPUT

TIAREF

TIA

OUTPUT

4.7kΩ

604Ω

6.7Ω

2kΩ

1.65V

REF

IPD

LTC6268

ADG772

10nF

100nF

22µF

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