When high-speed operational amplifiers process alternating AC signals powered by a single supply rail only, their output swing cannot extend below 0 V. The negative half-cycle of the AC waveform will be directly clipped, making full signal reconstruction impossible. For precision low-signal acquisition applications such as photoelectric detection and high-impedance weak sensor measurement, a dual-supply architecture with a dedicated negative voltage rail (typically −2.5 V) is mandatory to achieve full-swing linear amplification of AC waveforms.
Example: The LTC6268 is a wideband precision op-amp designed for fA-level tiny current sensing circuits, which impose stringent requirements on noise, ripple and stability of the negative supply rail. The quality of the negative power supply directly determines the overall signal acquisition accuracy of the equipment. Distortion or excessive noise on the negative rail will submerge faint photoelectric signals under noise, rendering all measurement data completely invalid.
LTC6268 Photodiode Transimpedance Amplifier (TIA)
(Images sourced from Analog Devices, ADI)
This is the standard photodiode Transimpedance Amplifier (TIA) circuit for the LTC6268 provided by ADI.
1. Power Supply Scheme (Core Function of Negative Voltage Rail)
- Op-amp positive supply: +2.5 V; non-inverting input tied directly to 0 V system ground.
- Op-amp negative supply: −2.5 V (the negative rail emphasized throughout this document).
Photodiode (PD)
The anode connects to +2.5 V and the cathode to the op-amp’s inverting input, operating under reverse bias. This configuration drastically reduces the diode junction capacitance and extends circuit bandwidth.
Incident light generates photocurrent \(I_{PD}\), which flows from the PD cathode into the op-amp’s inverting pin. This is the weak input signal of the circuit, ranging from pA to nA magnitude.
LTC6268 High-Speed Precision Op-Amp
Key device characteristics tailored to this circuit:
- FET input stage with fA-range input bias current, avoiding shunting of tiny photocurrent;
- Wide bandwidth: the closed-loop bandwidth reaches 65 MHz with the 20 kΩ feedback resistor configuration;
- Ultra-low input noise current, optimized for faint photoelectric detection;
- Requires the −2.5 V negative rail to support negative output swing.
20 kΩ Feedback Resistor (Core Transimpedance Component)
Function: Linearly convert photocurrent \(I_{PD}\) into output voltage with a transimpedance gain of 20 kΩ.
Parasitic Feedback Capacitance
Parasitic capacitance arises from resistor terminals, PCB traces and photodiode junction capacitance in parallel with the feedback resistor. Together with the 20 kΩ resistor, it forms an RC low-pass filter that narrows bandwidth and triggers high-frequency oscillation. A compensation capacitor must be added in practical designs to counteract parasitic effects.
Related Document
Related Part Number
Further Reading on Negative Voltage Fundamentals
- Negative Voltage Basics: What Is Negative Voltage?
- Negative Voltage Basics: Voltages in Communication Circuits
- Negative Voltage Basics: Why Op-Amp & Audio Analog Circuits Require Positive/Negative Power Supplies
- Negative Voltage Basics: Charge Pump Implementations
- Negative Voltage Basics: Positive and Negative Voltage Design for Buck Circuits
- Negative Voltage Basics: Negative Rail Supply for Precision Op-Amp AC Signal Amplification