Carrier Based Current Monitor

This circuit is utilizes AC carrier modulation techniques to meet APD current monitor requirements. This circuit features 0.4% accuracy over the sensed current range, runs from a 5V supply and has the high noise rejection characteristics of carrier based “lock in” measurements. This is the figure of the circuit.

This circuit is based on The LTC1043 that is used to switch array is clocked by its internal oscillator. Oscillator frequency, set by the capacitor at Pin 16, is about 150Hz. S1 clocking biases Q1 via level shifter Q2. Q1 chops the DC voltage across the 1kW current shunt, modulating it into a differential square wave signal which feeds A1 through 0.2mF AC coupling capacitors. A1’s single-ended output biases demodulator S2, which presents a DC output to buffer amplifier A2. A2’s output is the circuit output. Switch S3 clocks a negative output charge pump which supplies the amplifier’s V– pins, permitting output swing to (and below) zero volts. The 100k resistors at Q1 minimize its on-resistance error contribution and prevent destructive potentials from reaching A1 (and the 5V rail) if either 0.2mF capacitor fails. A2’s gain of 1.1 corrects for the slight attenuation introduced by A1’s input resistors. In practice, it may be desirable to derive the APD bias voltage regulator’s feedback signal from the indicated point, eliminating the 1kW shunt resistor’s voltage drop. [Schematic’s circuit source: Linear Technology Notes].

Trans-Impedance Amplifier for DAC Outputs Using LH4117 Single IC Chip

This is a circuit for trans-impedance amplifier that using to DAC output. This circuit is based on LF4117 that is an excellent match to amplify the output of DACs like the DAC0800. The fast settling time of 9 ns to 0.02% does not degrade the performance of the DAC. On the other hand, the DAC0800 provides complimentary current outputs, which are current sinks and do not need a fixed voltage to work accurately. This is the figure of the circuit.

The DAC0800 is fed a reference current of IREF e 2 mA into pin 14. This is achieved by the LH0070 voltage reference with 10V output. A resistance of 5 kX (R1 a R2) is connected to pin 14, which is a virtual ground, thus providing the reference current of 2 mA. The grounded pin 15 provides the reference voltage for pin 14. The DAC has eight current sinks, each set for half the current of the previous one. Through switches, controlled by the input logic levels, their open collectors are connected to one of the two outputs. The sum of the output currents I1 and I2 equals the reference current of 2 mA. Because of the open collector configuration the outputs do not have to be tied to a fixed voltage level. The outputs of the DAC0800 are connected to the inputs of the LH4117 through 100X resistors (R4 and R5). This is to decouple the inputs of the amplifier from the output capacitance of the DAC, which is typically 23 pF to 30 pF. Especially the inverting input is sensitive to capacitance. [Schematic’s circuit source: National Semiconductor Notes].