LINEAR/LOGARITHMIC CAPACITIVE TRANS-IMPEDANCE AMPLIFIER CIRCUIT
A dual mode amplifier for photodiode output reads a photodiode providing a current input. An integration capacitor is connected to receive the current input and connected to a voltage output node. An input capacitor is also connected to receive the voltage input and drives an operational amplifier having an output connect to the voltage output node. A correlated double-sample (CDS) capacitor is connected to the voltage output node and a sample and hold circuit incorporating a sample and hold capacitor for sampling the CDS capacitor. A log function on (LOGON) switch is connected between the operational amplifier output and the voltage output node and a reset switch connected to short the photodiode. The LOGON switch sequentially operates in conjunction with the reset switch at the end of an integration time allowing the integration capacitor to retain an integrated current plus a log voltage by closing of the reset switch shorting the detector photodiode and offsetting the voltage on the integration capacitor into the negative voltage direction.
1 . A dual mode amplifier for photodiode output comprising:
a photodiode providing a current/voltage input;
an integration capacitor connected to receive the current/voltage input and connected to a voltage output node;
an input capacitor connected to receive the voltage input and driving an operational amplifier having an output connect to the voltage output node;
a sample and hold circuit having a sample and hold capacitor for sampling the integration capacitor;
a log function on (LOGON) switch intermediate the operational amplifier output and the voltage output node;
a reset switch connected to short the photodiode;
said LOGON switch sequentially operable in conjunction with the reset switch at the end of an integration time allowing the integration capacitor to retain an integrated linear voltage plus a log voltage by closing of the reset switch shorting the detector photodiode and offsetting the voltage on the integration capacitor into the negative voltage direction.
2 . The dual mode amplifier for photodiode output of claim 1 further comprising a correlated double sample (CDS) capacitor connected to the voltage output node.
3 . The dual mode amplifier for photodiode output of claim 1 further comprising an auto-zero switch connected between the operational amplifier input and output and a CDS switch connected to the output of the CDS capacitor.
4 . A method for linear and logarithmic measurement of a diode detector comprising:
obtaining a current/voltage input from a diode detector;
integrating the current input on an integration capacitor;
inputting the current/voltage input through a capacitor to an operational amplifier;
charging a double-sample capacitor with an output from the operational amplifier and the integration capacitor;
interrupting the output of the operational amplifier after an integration period and sampling the linear input from the integration capacitor for a first sample period;
shorting the diode detector for sampling the logarithmic input for a second sample period; and
activating a sample and hold circuit to read the double-sample capacitor.
5 . The method of claim 4 further comprising resetting the amplifier and CDS capacitor.