ADA4356ABCZ datasheet(39/61 Pages) AD

39 / 61 page

Data Sheet

ADA4356

analog.com

Rev. 0

39 of 61

Switch On-Resistance

Real switches have nonzero on-resistance (RON), which appears in series with R1. Thus, switch on-resistance affects

the divider ratio of R1 and R2. The resulting current division with nonzero switch RON is given by:

I2 =

(R1 + RON)

(R1 + RON)+R2

IPD

In the special case where R1 = 0Ω, this equation reduces to:

I2 =

RON

RON + R2

IPD

The switch nonideality RON thus effectively plays the role of the resistor R1 in the original divider in Figure 81.

Note that RON of a switch has a different temperature coefficient than that of a discrete resistor, which affects

current division accuracy over temperature. Calibration over temperature can help address such accuracy issues.

Switch Current Limits

The maximum input current that the current divider can accept is limited by switch S1’s absolute maximum current

limits. The absolute maximum current limits for current pulses are often much higher than the limit for continuous

currents.

Aside from absolute maxima for switch currents, another issue with large input currents I1 is the resulting larger

transient IR drop due to the switch on-resistance (RON × I1). Since the TIA input is pinned at 1.65V, this IR drop

pushes the output of the switch itself higher. An IR drop that is too big may force the switch’s output too close to its

own supply voltage, which can cause nonlinearity errors.

Additionally, self-heating from high currents through S1 also causes temperature coefficient shifts in RON.

Switch Capacitance

While a larger switch has the benefits of higher current capacity and a smaller switch on-resistance RON, the trade-

off is an increased switch parasitic capacitance CSW. This capacitance also appears at the input of the TIA, which can

affect TIA performance as explained in the Transimpedance Amplifier Input section.

Although resistors R1 and R2 may appear to isolate the INPUT node from the total input capacitance CS and switch

capacitance according to typical nodal analysis rules, this assumption does not apply here, because the TIA input is

a current, not a voltage. A resistor cannot increase or decrease current, while any capacitance on the path between

input source (that is, APD) and ball INPUT (E1) can divert desired input current.

Capacitor C1 is the sole exception, because it is so large that its associated poles and zeros are multiple decades

below the frequencies of interest for this application. It appears solely as an AC ground and does not affect TIA

performance. Meanwhile, switch capacitance CSW is typically in the pF range, and thus its associated poles and

zeros are close enough to the TIA’s own poles and zeros to affect bandwidth and noise.

A recommended switch that balances low on-resistance and low parasitic junction capacitance is ADG772.

OTDR Application

The small form factor and integrated nature of the ADA4356 micromodule make it ideally suited for space-sensitive

applications, such as embedded optical time domain reflectometry (OTDR) for fiber optic cable installations in

datacenters or telecommunications networks.

The 133kΩ transimpedance gain and 1MHz LPF cutoff frequency enables the ADA4356 to reach the high sensitivity

and low noise levels needed for wide dynamic range long-haul OTDR applications. Conversely, the 4.54kΩ gain and

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