10Gbps, 3.3V Low-Power Transimpedance

Amplifier with RSSI

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Detailed Description

The MAX3970 transimpedance amplifier is optimized

for 10Gbps fiber optic receivers. Figure 1 is a function-

al diagram of the MAX3970, which comprises a tran-

simpedance amplifier, a voltage amplifier, an output

buffer, a received-signal strength indicator, and a DC-

cancellation circuit.

Transimpedance Amplifier

Photodiode signal current flows into the summing node

converts this current into a voltage with a gain of

approximately 400

Ω. Schottky diodes clamp the output

voltage for large input currents, as shown in Figure 2.

Voltage Amplifier

The voltage amplifier converts single-ended signals to

differential signals and introduces approximately 4dB

of gain.

Output Buffer

The output buffer is optimized to drive a 100

Ω differential

load between OUT+ and OUT-. Although short-circuit

protection is provided, this stage will not drive a 50

Ω

load to ground. For proper operation, the load must be

AC-coupled. For large signals, the output buffer

Terminate the MAX3970 outputs differentially for optimum

supply-noise rejection. If a single-ended output is

required, terminate the used and unused outputs similarly.

DC Cancellation Circuit

The DC cancellation circuit centers the input signal

within the transimpedance amplifier’s linear range

(Figure 3). Low-frequency feedback is employed to

remove the input signal’s DC component.

The DC cancellation circuit is internally compensated

and therefore does not require external capacitors. This

circuit minimizes pulse-width distortion for data

sequences that exhibit a 50% mark density. A mark

density significantly different from 50% will cause the

MAX3970 to generate pulse-width distortion.

Received-Signal Strength Indicator

The received-signal strength indicator (RSSI) provides a

voltage proportional to the DC input current. The RSSI

circuitry is designed to drive a 10k

Ω load and is used

during the assembly process to optimally align the pho-

todiode. The lowpass filter in the DC cancellation circuit

determines the response time of the RSSI circuit.

Design Procedure

Power Supply

The MAX3970 requires wide-band power-supply

decoupling. Power-supply bypassing should provide

cies between 50kHz and 10GHz. Use LC filtering at the

main supply terminal and decoupling capacitors as

close to the die as possible.

PAD

NAME

FUNCTION

BP1, BP2,

BP18

Power Supply. Provides supply voltage to input circuitry and bias to the photodiode via an

internal 410

Ω resistor.

BP3

FILTER

Provides bias voltage for the photodiode through a 410

disables the DC cancellation circuit to allow a DC path from IN to OUT+ and OUT- for testing.

BP4

TEST

Test Pad. This pad is connected to IN via a 1k

Ω resistor.

BP5

IN

Amplifier Input. Accepts photodiode input current.

BP6, BP7

GND1

Ground

BP8, BP9

GND2

Ground

BP10, BP13

GND3

Ground

BP11

OUT-

Negative CML Output. Current flowing into IN causes OUT- to decrease.

BP12

OUT+

Positive CML Output. Current flowing into IN causes OUT+ to increase.

BP14, BP15

BP16

Power Supply. Provides supply voltage to the output buffers.

BP17

RSSI

Received-Signal Strength Indicator. This pin provides a voltage proportional to the DC input

current. Monitor this output during assembly to optimally align the photodiode to the optics.

Pad Description