The differential outputs of the LMH32401 device can directly drive a high-speed differential input ADC. Figure 8-1 shows the LMH32401 differential outputs directly driving the ADC12QJ1600. The effective signal gain between the TIA input and the ADC input is 2 kΩ or 20 kΩ when driving an ADC with a 100-Ω differential input impedance (R_{ADC_IN} = 50 Ω). Equation 2 gives the effective signal gain between the TIA input and the ADC input when driving an ADC with any other value of differential input impedance (R_{ADC_IN} ≠ 50 Ω).

Figure 8-1 LMH32401 to ADC Interface

Equation 2.

Where:

• A_V = Differential gain from the TIA input to the ADC input
• R_{ADC_IN} = Input resistance of the ADC

Figure 8-2 shows a matching resistor network between the LMH32401 output and the ADC12QJ1600 input. The matching network is needed to prevent signal reflections when the signal path between the LMH32401 and ADC is very long. Equation 3 gives the effective gain from the TIA input to the ADC input when using a matching resistor network.

Figure 8-2 LMH32401 to ADC Interface with a Matching Resistor Network

Equation 3.

Where:

• A_V = Gain from the TIA input to the ADC input
• R_{ADC_IN} = Differential input resistance of the ADC
• R_ISO = Series resistance between the TIA and ADC

Equation 4 gives the voltage to be applied at the VOD pin (pin 9) if a certain differential offset voltage (V_OD) is needed at the ADC input for the circuit in Figure 8-2.

Equation 4.

Where:

• VOD = Voltage applied at pin 9
• V_OD = Desired differential offset voltage at the ADC input
• R_{ADC_IN} = Differential input resistance of the ADC
• R_ISO = Series resistance between the TIA and ADC