SNOAA83 December 2021 TLV3601 , TLV3601-Q1 , TLV3603 , TLV3603-Q1
One of the goals of this design is to operate from a single, 5-V supply. This design uses a three-resistor divider network to establish the common-mode output voltage and the comparator reference voltage.
The important thing to note for this resistive divider network is to consider the input bias currents of both the OPA858 and TLV3601 devices. Since the OPA858 has an ultra-low bias current of 10 pA, the largest source of error comes from the TLV3601. The input bias current of the TLV3601 is typically 1 μA which means that the current through the divider network should be at least 100 times larger to maintain the desired reference voltages. With a 5-V supply and a current of 100 μA, the maximum total resistance for this network is 50 kΩ.
For this design, the common-mode voltage of the OPA858 is set to 500 mV, a bias voltage within the recommended common-mode range for the OPA858. To do this, divide 500 mV by the 100 μA desired divider current. This gives a value for R3 of 5 kΩ but 4.99 kΩ was used for this design.
To comply with the design requirements, the OPA858 output will swing 100 mV. With the 500-mV output common-mode established, the comparator threshold voltage must be in the range of 500 mV to 600 mV. The TLV3601 threshold is 575 mV for this design. To provide an additional 75 mV from the 500-mV reference, R2 must be 750 Ω with the total branch current still being 100 μA.
To adhere to the maximum resistance and minimum current requirement, R1 was selected to be 44.2 kΩ. This gives a total resistance of 49.94 kΩ.
With a 100-μA pulse of current through the feedback branch of the OPA858, a 1-kΩ feedback resistance is required to produce a 100-mV swing on the output.
For this application, a 3-ns light pulse is received as a 100-μA current pulse. Assuming at most one, 3-ns pulse in a 10-ns window, the total period of our input is 10 ns. A 10-ns period corresponds to a 100-MHz signal. To select the feedback capacitor, first consider the pole frequency of a feedback network with a capacitor and resistor in parallel. The rough pole frequency is expressed as follows:
With a 1-pF capacitor in the feedback loop and a 1-kΩ feedback resistor, the pole frequency is approximately 159 MHz. The input signal is within the bandwidth of the feedback impedance. Additional stability analysis is also required for the transimpedance amplifier circuit and the metrics used to check for stability were rate of closure (ROC) and phase margin. For further information on stability analysis see the Op Amps: Stability - Phase Margin and Op Amps: Stability - Spice Simulation TI Precision Labs training videos.
To mimic the behavior of a photodiode receiving a 3-ns pulse of light, a piecewise current generator is configured to pulse 100 μA for 3 ns in a 10-ns period. The parallel input capacitance is set to 1 pF. For more information on a photodiode equivalent model see the 1 MHz, Single-Supply, Photodiode Amplifier Reference Design.
This design uses the TLV3601 high-speed comparator in a non-inverting configuration. To configure the comparator, connect the voltage node above R2 to the inverting input and designate it VREF. Connect the same 5-V supply used for the OPA858 and connect the VEE pin to ground. The input common-mode range with a 5-V supply is –0.3 V to 5.3 V. With one of the inputs swinging from 500 mV to 600 mV and VREF being 569.6 mV, both inputs adhere to the input common-mode range of the TLV3601. If extra hysteresis is required to avoid output chatter due to noise or input signal conditions, then use the TLV3603. The TLV3603 has an extra hysteresis pin if hysteresis is required for an application.