MAT03

–9–

REV. B

the total output impedance is:

Figure 16. Cascode Current Source

CURRENT MATCHING

The objective of current source or mirror design is generation of

currents that are either matched or must maintain a constant ra-

tio. However, mismatch of base-emitter voltages cause output

current errors. Consider the example of Figure 17a. If the resis-

tors and transistors are equal and the collector voltages are the

same, the collector currents will match precisely. Investigating

define

∆I

Graph 17b describes the relationship of current matching errors

since the relative error between the currents is exponentially pro-

portional to the offset voltage, tight matching is required to de-

sign high accuracy current sources. For example, if the offset

voltage is 5 mV at 100

µA collector current, the current match-

ing error would be 20%. Additionally, temperature effects such

as offset drift (3

µV/°C per mV of V

DIGITALLY PROGRAMMABLE BIPOLAR CURRENT

PUMP

The circuit of Figure 18 is a digitally programmable current

pump. The current pump incorporates a DAC08, and a fast

Wilson current source using the MAT03. Examining Figure 18,

the DAC08 is set for 2 mA full-scale range so that bipolar cur-

rent operation of

±2 mA is achieved. The Wilson current mirror

maintains linearity within the LSB range of the 8-bit DAC08

(

±2 mA/256 = 15.6 µA resolution) as seen in Figure 19. A

current so that it matches the reference current programmed by

the DAC08.

independent of the voltage across the current source output.

the switching speed of the current mirror. The feedback serves

to increase the output impedance and improves accuracy by re-

ducing the base-width modulation which occurs with varying

collector-emitter voltages. Accuracy and linearity performance

of the current pump is summarized in Figure 19.

Figure 17a. Current Matching Circuit

Figure 17b. Current Matching Accuracy %

vs. Offset Voltage

Figure 18. Digitally Programmable Bipolar Current Pump