Patent Application
20140111188
This invention relates generally to electrical measurement equipment and, in particular, to an active shunt ammeter for use in measuring electrical current.
Source measure units (SMU) are used to make precision measurements in many fields, including the testing of semiconductor products. For example, U.S. Pat. No. 5,039,934 describes one such device and range-changing in such a device is described in U.S. Pat. No. 5,144,154, both of which are incorporated herein by reference in their entireties. Typical SMU designs include a voltage or current source with integrated voltage and current measurement capabilities. A device under test (DUT) is coupled to the SMU and is then stimulated with either the voltage or current source.
There are several ways in which the current through a DUT may be measured. For example, a shunt ammeter, may be used to simply sense the voltage across a resistor RS. RS must be kept small to not cause a large burden voltage to the input signal. A low noise gain stage is required to amplify the burden voltage so it can be measured.
A feedback ammeter uses a high gain op-amp to pull the input circuit through the resistor RS. The op-amp keeps the burden voltage low because of its high dc gain (typically greater than 1 million). This allows RS to be larger allowing the output signal to be larger. However, the op-amps high gain begins to roll off at relatively low frequencies. This causes the burden voltage to increase at higher frequencies as well. If the input is capacitive, it can cause the feedback ammeter to ring or even oscillate. It would be desirable to provide improved ammeter configurations that address these problems.
An active shunt ammeter for measuring current flowing through a device under test (DUT) and method are disclosed. The active shunt ammeter includes an input configured to receive an input signal having a frequency within a frequency band and representing the current flowing through the DUT. An output is configured to generate an output voltage representing the current flowing through the DUT. The active shunt ammeter also includes a gain circuit having an amplifier with a gain characteristic that varies respect to frequency within the frequency band and a feedback element having an impedance coupled from an output of the gain circuit to a negative input of the gain circuit, the feedback element impedance being configured to change with frequency to correlate with the amplifier gain characteristic such that the feedback element impedance divided by the amplifier gain over the frequency band has minimal frequency dependency.
The amplifier may have a parallel RC feedback element. The amplifier may be a differential amplifier with a parallel RC feedback element coupled between a negative-input terminal and an output terminal. The gain circuit may have an input impedance that remains generally constant across the entire bandwidth of the amplifier based on the gain characteristic and the feedback element impedance. The amplifier may have a controlled negative gain across the feedback element. The amplifier may have an inverting stage having a gain set by a resistor ratio. The amplifier may have a gain that is split between two operational amplifiers (op-amps). A voltage across the feedback element may be buffered and attenuated by a resistor ratio. The amplifier may have an input op-amp with a gain placed in its feedback path.
A method of measuring current flowing through a device under test (DUT) is also disclosed, the method includes receiving an input signal having a frequency within a frequency band and representing the current flowing through the DUT. An output voltage is generated, the output voltage representing the current flowing through the DUT. A gain circuit is provided. The gain circuit has an amplifier with a gain characteristic that varies respect to frequency within the frequency band and a feedback element having an impedance coupled from an output of the gain circuit to a negative input of the gain circuit, the feedback element impedance being configured to change with frequency to correlate with the amplifier gain characteristic such that the feedback element impedance divided by the amplifier gain over the frequency band has minimal frequency dependency.
The amplifier may have a parallel RC feedback element. The amplifier may be a differential amplifier with a parallel RC feedback element coupled between a negative-input terminal and an output terminal. The gain circuit may have an input impedance that remains generally constant across the entire bandwidth of the amplifier based on the gain characteristic and the feedback element impedance. The amplifier may have a controlled negative gain across the feedback element. The amplifier may have an inverting stage having a gain set by a resistor ratio. The amplifier may have a gain that is split between two operational amplifiers (op-amps). A voltage across the feedback element may be buffered and attenuated by a resistor ratio. The amplifier may have an input op-amp with a gain placed in its feedback path.
The disclosure herein relates generally to electrical measurement equipment and, in particular, to an active shunt ammeter for use in measuring electrical current. Such ammeters are often a sub component of measurement products including digital multi-meters (DMM) and source measure units (SMU). There are several ways in which the current through a device under test (DUT) may be measured.
An active shunt ammeter design addresses these problems. An active shunt ammeter configuration generally replaces the op-amp used in the feedback ammeter with a fixed gain amplifier. The result is a gain that is constant to higher frequencies. At the frequency the amplifier begins to roll off, the capacitor impedance (1/jωCs) is designed to equal RS. The roll off of the parallel impedance of RS and Cs combined with the roll off the amplifier's gain, results in an input-impedance of the ammeter that is constant across the entire bandwidth of the amplifier. The result is a shunt like ammeter with higher output signal vs. burden voltage than a traditional shunt ammeter and none to the stability issues of feedback ammeters.
In
If the feedback element 32 was resistive only, i.e., capacitor 36 was omitted, the input impedance Zin would increase with frequency after the target frequency 52. The impedance of capacitor 36 may be selected to equal the impedance of the resistor at the target frequency 52. This causes the impedance of the feedback element 32 to drop at the same frequency the operational amplifier 40 begins to roll off. This configuration yields a flat input impedance that does not roll off after the target frequency 52 as shown in
It should be understood that a fixed gain amplifier may be implemented in several configurations.
It should be understood that many variations are possible based on the disclosure herein. Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements.
