ADA4356ABCZ datasheet(37/61 Pages) AD

37 / 61 page

Data Sheet

ADA4356

analog.com

Rev. 0

37 of 61

If buffer A1 is not present, a charge continues to accumulate on C1 with each successive input current pulse, pushing

the voltage at node V1 up to the supply. On its own, the C1 discharges far too slowly for the AC ground at V1 to return

to 1.65V in time for the next pulse on IPD, which violates the underlying assumption of the current divider that

INPUT and TIAREF are always at the same DC bias.

Figure 82. Simulated Input Current Pulse IPD and Resulting Voltage Perturbation at Node V1

Figure 83. Simulated Input Current Pulses IPD and AC Ground Settling Response at V1

30mA Current Divider Example

Figure 84 shows an example circuit that can divide down a maximum DC input current of 30mA from IPD to fit within

the 300µA linear input current range of the ADA4356.

Capacitor C1 is 22.11µF, from a combination of 22µF, 100nF, and 10nF capacitors in parallel. Resistor R1 is the on-

resistance of the chosen switch S1, ADG772, which is roughly 5Ω at room temperature and with a supply voltage of

3.3V. Resistor R2 is a discrete 604Ω. The resulting current division is:

I2 =

RON

RON + R2

IPD=

5Ω

5Ω + 604Ω

IPD = 0.00821 × IPD≈

IPD

120

Thus, I2 = 30mA/120 = 250µA, which fits within the linear input range of 320µA.

The rest of the current, which is diverted into the AC ground at V1, is I1 = 30mA – I2 = 30mA – 166µA = 29.833mA.

IPD

TIME (µs)

0.05

0.10

0.15

0.20

0.25

1.64996

1.64998

1.65000

1.65002

1.65004

1.65006

–10

IPD

TIME (µs)

1.64996

1.64998

1.65000

1.65002

1.65004

1.65006

–10

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