The LMH32401-Q1 features a programmable gain transimpedance amplifier (TIA) stage followed by a fixed-gain, single-ended input to differential output amplifier stage. The closed-loop bandwidth and noise of a TIA are affected by the transimpedance gain and photodiode capacitance. For a given value of photodiode capacitance, the LMH32401-Q1 has higher bandwidth in the device low-gain configuration compared to the high-gain configuration. Increasing the gain of the TIA stage by a factor of X increases the output signal by a factor X, but the noise contribution from the resistor only increases by √X. The input-referred noise density of the low-gain configuration is therefore higher than the input-referred noise density of the high-gain configuration.

The gain of the TIA stage is controlled by the GAIN pin. Setting this pin low places the TIA in the low-gain configuration; whereas, setting the pin high places the TIA in a high-gain configuration. The LMH32401-Q1 defaults to the low-gain configuration when the GAIN pin is left floating.