Many single supply powered applications require amplifier output swings within millivolt or even sub-millivolt levels of ground. Amplifier output saturation limitations normally preclude such operation. Figure 1's power supply bootstrapping scheme achieves the desired characteristics with minimal component addition.
A1, a chopper stabilized amplifier, has a clock output. This output switches Q1, providing drive to the diode-capacitor charge pump. The charge pump output feeds A1's V- terminal, pulling it below zero, permitting output swing to (and below) ground. If desired, the negative output excursion can be limited by either clamp option shown.
Reliable start-up of this bootstrapped power supply scheme is a valid concern, warranting investigation. In Figure 2, the amplifier's V- pin (trace C) initially rises at supply turn-on (trace A) but heads negative when amplifier clocking (trace B) commences at about midscreen.
The circuit provides a simple way to obtain output swing to zero volts, permitting a true "live at zero" output.
From the datasheet
The LTC1150 used in this application is actually a CMOS chopper-stabilized amplifier. It includes two internal amplifiers: a fast amp, which processes signals directly to the output, and a dc-stabilizing amp, which periodically samples the offset of the fast amp and maintains its output-hold capacitor at a value that minimizes offset errors. The two sample-and-hold capacitors usually required externally by other chopper amplifiers are integrated on-chip.
The LTC1150 has an offset voltage of 0.5uV, drift of 0.01uV/C, 0.1Hz to 10Hz input noise voltage of 1.8uVP-P and a typical voltage gain of 180dB. The slew rate of 3V/us and a gain bandwidth product of 2.5MHz are achieved with 0.8mA of supply current. Overload recovery times from positive and negative saturation conditions are 3ms and 20ms, respectively. The LTC1150 can be a plug-in replacement for most standard bipolar op amps. For applications demanding low power consumption, Pin 1 can be used to program the supply current.
The minimum supply for proper operation of the LTC1150 is typically below 4.0V (2.0V). In single supply applications, PSRR is guaranteed down to 4.7V (2.35V) to ensure proper operation down to the minimum TTL specified voltage of 4.75V.
Applications for this part include signal conditioning circuits for low-level photodetectors, and battery discharge monitors.
Figure 1. Single rail powered amplifier has true zero volt output swing. A1's clock output switches Q1, driving diode-capacitor charge pump. A1's V- pin assumes negative voltage, permitting zero (and below) volt output swing.
Figure 2. Amplifier bootstrapped supply start-up. Amplifier V- pin (trace C) initially rises positive at 5V supply (trace A) turn-on. When amplifier internal clock starts (trace B, 5th vertical division), charge pump activates, pulling V- pin negative.