AN3428| Application Note
Maxim/Dallas > App Notes > MEASUREMENT CIRCUITS
SENSOR SIGNAL CONDITIONERS
Keywords: current transformers, transimpedance amplifiers, current measurement
Dec 01, 2004
APPLICATION NOTE 3428
Transimpedance Amplifier Buffers Current Transformer
Abstract: A general-purpose current-measurement system employs a current transformer, ac-coupled to a transimpedance amplifier. About transimpedance and transconductance: The words "transconductance" and "transimpedance" are often used interchangeably. Technically, the terms differ: a transimpedance amp delivers an output voltage that is a function of the input current; conversely, a transconductance amp converts a voltage to a current. Current transformers are not only a convenient means for current measurement in many applications, but also provide isolation while absorbing very little power from the source. The output current from a current transformer can be converted to voltage with a suitably valued resistor, but that approach can dramatically reduce the transformer's high-frequency response. Transimpedance amplifiers are a good method for converting current to voltage in most current-measurement applications. The current source feeds into the virtual ground of an op amp, and the transimpedance can be
adjusted by changing the value of a single resistor (Figure 1). As another benefit of this arrangement, the circuit gain for input offset voltage is no greater than unity.
Figure 1. For resistor values less than 20M current sources.
, this simple transimpedance amplifier is acceptable for use with some
are notoriously difficult to apply. For applications requiring a transimpedance above Resistor values above 20M 20M , consider the circuit of Figure 2. The output voltage-divider adds voltage gain that multiplies the transimpedance value, enabling effective transimpedances greater than 200M . Note that the gain for input offset voltage now equals the additional voltage gain.
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