Femtoamp-bias op amps reach 500MHz gain-bandwidth

October 01, 2014 // By Graham Prophet
LTC6268 and LTC6269 are FET-input single and dual op amps, for high speed and high dynamic range transimpedance amplifier (TIA) and buffer applications. With 3 fA bias current at 25°C and 4 pA max over the entire -40°C to 125°C temperature range, the LTC6268/LTC6269 can resolve input currents from several femtoamps to amps.

Wideband voltage and current noise is 4.3 nV/√Hz and 5.5 fA/√Hz, respectively, enabling high dynamic range circuits. For speed as well as precision, the LTC6268 features 500 MHz gain bandwidth product, sub-pF input capacitance and -100 dB harmonic distortion at 1 MHz. -3dB bandwith (A = 1) is 350 MHz; input capacitance is 450 fF and the amplifiers' slew rate is 400 V/µsec. In addition to photodiode and photomultiplier (PMT) circuits, the LTC6268/9 are well suited for other high impedance sensor applications and for driving analogue-to-digital converters (ADCs).

The LTC6268 data sheet shows circuit and layout techniques to maximise TIA performance, with example circuits ranging from 20 kΩ transimpedance gain at 65 MHz bandwidth, to 499 kΩ transimpedance gain at 11.2 MHz bandwidth.

The LTC6268 is available in a 6-lead SOT-23 package, as well as an 8-lead SOIC with guard pins to protect against board leakage currents. The LTC6269 is available in MSOP-8 with exposed pad and 3 x 3 mm DFN-10 packages. Both are unity-gain stable and fully specified over the industrial -40°C to 85°C and -40°C to 125°C temperature ranges. Prices start at $2.90 for the LTC6268 and $4.90 for the LTC6269 (1000).

Linear Technology; www.linear.com/product/LTC6268 / www.linear.com/product/LTC6269