Ground impedance current sensor reconstructs input current
Reconstructing a small signal current flowing into a ground impedance that is part of a larger circuit. This current can be the sum of multiple currents, for example, it is the high pass filtered output of several current mode quad filters flowing into one capacitor (and is an operating impedance of the filter).
Using a current mirror, a single copy of all the input currents can reconstruct the output current, but it is prone to error, as shown in the following equation: IIN = I1 + I2+ I3+... +IN, and IO = α1I1 + α2I2 + α3I3 + ... + αNIN. To accurately reconstruct the IIN, the ideal current mirror factor should be a unit value. However, all of these coefficients not only deviate from the ideal unit value, but may also differ due to the mismatch of the current mirror, including the systematic mismatch of the finite current impedance of the current mirror and the random threshold mismatch of the CMOS current mirror. . These mismatches can cause changes in the small signal current frequency characteristics (eg, high-pass response) at the output. One technique that can be used is to sense the voltage across the impedance and then make a single-precision voltage-current conversion to avoid errors caused by multiple mirroring operations.
The current mirror reflects the current drawn by the load Z2, and the load current (and its current) matches the impedance to be measured Z1.
The figure shows a small signal current sensing circuit that must detect the total current flowing through an impedance Z1. The voltage-current conversion requires a matched load impedance Z2 and is built around an operational current transmitter. Any mismatch between the loads will cause the output current to be equal to a certain scaling value of the input current. The scaling factor is the ratio of impedances, so the frequency characteristics of the small signal output current remain the same.
The mismatch in the two current mirrors is also related: assuming that the input current of current mirror 1 is IB+ IIN and the input current of current mirror 2 is IB, the output current is IO=β1IB+β1IIN-β2IB=(β1-β2) IB+ β1IIN, where IB is the quiescent current of the last stage of the amplifier, and β1 and β2 are the mismatch coefficients of the current mirror. The output current has a dc bias current that is easily removed, and there is also a scaling value for the input small signal current IIN.
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