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Use current-sink, current-source ICs as current mirrors

Posted: 30 Jan 2009     Print Version  Bookmark and Share

Keywords:current sinks  current mirror  MAX4123 

The 4-20 mA transmitters used in industrial current-control loops can be implemented as powered current sources, remote non-powered current sinks, or with several other IO combinations.

Figure 1: A unity-gain, input-sink/output-sink current mirror.

A useful building block for some of these applications is a precision current mirror, which allows you to transform one type of transmitter into another, or to create a repeater for the purpose of extending the loop length. Circuit examples include an input-sink output-sink mirror (Figure 1), and an input-sourceoutput-source mirror (Figure 2). These circuits have similar configurations but differ slightly in performance. You can also build input-sinkoutput-source mirrors and input-source output-sink mirrors by chaining one circuit type to the other.

Figure 2: A unity-gain, input-source/output-source mirror.

The circuits have output impedances in the range of 10 O to O, and a current-mirror accuracy that is defined (for Figure 1) by the precision of its matched-resistor ratio. The use of resistors matched to 0.1 percent, for instance, produces a mirroring accuracy equivalent to 10-bit resolution. Within a range of values, the absolute value of these matched current-sensing resistors has no effect on the mirroring accuracy. The value shown (30.1 O) is somewhat arbitrary, and you can increase it at the cost of an added input drop and an increase in the minimum operating output voltage.

With sense-resistor values as shown, the Figure 2 circuit adds an offset uncertainty of one 10-bit LSB at full scale, due to the larger offset of the MAX4123 (600 microvolts) vs. that of the MAX4236 (20 microvolts) used in Figure 1a. The use of higher-valued sense resistors reduces this uncertainty.

The compliance range for operating voltage extends from a minimum of 4 V (both circuits) to a maximum that is slightly lower for the Figure 1 circuit (90 V), because of its different output device. To configure either of these circuits as a current amplifier, you simply adjust the current gain by altering the ratio of the current-sense resistors.

By Alfredo H. Saab and Shasta Thomas
Maxim Integrated Products

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