Linear Tech is aiming at high-impedance sensor transimpedance amplifiers with an op-amp that requires only 3fA of bias current (typical at 25°C) while offering 500MHz gain bandwidth product.
Potential applications for the chip, called LTC6268, include interfacing ADCs to reverse leakage (linear) mode photodiodes. 20kΩ transimpedance gain is available at 65MHz, and 499kΩ with 11MHz bandwidth.
FET input transistors are responsible for the low leakage, plus attention to pin protection.
“The LTC6268 uses a special bootstrapped ESD diode circuit to maintain bias current at 900fA max at 85°C and 4pA max at 125°C. This makes high dynamic range measurements possible, even in a variable temperature environment,” said the firm.
With such high impedances, guard band PCB layout is advisable, and the SOIC version of the chip has unconnected pins either side of the inverting input to allow a guard ring to be implemented (see diagram) – the data sheet includes circuit and layout tips.
Typical input capacitance is 0.45pF, adding little to the front-end circuit, and input-referred noise 4.3nV/√Hz or 5.5fA/√Hz.
Harmonic distortion is -100dB (1MHz 2Vp-p), and -80dB at 10MHz.
The input stage is remarkably complex with two separate stages following a CMOS buffer: one covering the negative rail to within 1.55V of the positive rail, where the other takes over, working to within 0.5V of the positive rail. Input offset is 200µV typical and ±2.5mV max (40°C to 125°C, rising to ±4.5mV when the common-mode input range reaches 4V and both input stages are in action.
Maximum input voltage difference is 2V, beyond which protection switches in. Max allowable input current is 1mA.
For more detail: Sub-picoamp input op-amp offers high gain bandwidth