AD8628ART datasheet(18/24 Pages) AD

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AD8628/AD8629/AD8630

Rev. I | Page 18 of 24

PRECISION CURRENT SHUNT SENSOR

OUTPUT AMPLIFIER FOR HIGH PRECISION DACS

A precision current shunt sensor benefits from the unique

attributes of auto-zero amplifiers when used in a differencing

configuration, as shown in Figure 63. Current shunt sensors are

used in precision current sources for feedback control systems.

They are also used in a variety of other applications, including

battery fuel gauging, laser diode power measurement and control,

torque feedback controls in electric power steering, and precision

power metering.

The AD8628/AD8629/AD8360 are used as output amplifiers for

a 16-bit high precision DAC in a unipolar configuration. In this

case, the selected op amp needs to have a very low offset voltage

(the DAC LSB is 38 μV when operated with a 2.5 V reference)

to eliminate the need for output offset trims. The input bias

current (typically a few tens of picoamperes) must also be very

low because it generates an additional zero code error when

multiplied by the DAC output impedance (approximately 6 kΩ).

0.1Ω

SUPPLY

100Ω

100kΩ

100Ω

100kΩ

e = 1000 RS I

100mV/mA

AD8628

Rail-to-rail input and output provide full-scale output with very

little error. The output impedance of the DAC is constant and

code independent, but the high input impedance of the AD8628/

AD8629/AD8630 minimizes gain errors. The wide bandwidth

of the amplifiers also serves well in this case. The amplifiers,

with settling time of 1 μs, add another time constant to the

system, increasing the settling time of the output. The settling

time of the AD5541 is 1 μs. The combined settling time is

approximately 1.4 μs, as can be derived from the following

equation:

()

(

) (

AD8628

DAC

TOTAL

Figure 63. Low-Side Current Sensing

In such applications, it is desirable to use a shunt with very low

resistance to minimize the series voltage drop; this minimizes

wasted power and allows the measurement of high currents

while saving power. A typical shunt might be 0.1 Ω. At measured

current values of 1 A, the output signal of the shunt is hundreds

of millivolts, or even volts, and amplifier error sources are not

critical. However, at low measured current values in the 1 mA

range, the 100 μV output voltage of the shunt demands a very

low offset voltage and drift to maintain absolute accuracy. Low

input bias currents are also needed, so that injected bias current

does not become a significant percentage of the measured current.

High open-loop gain, CMRR, and PSRR help to maintain the

overall circuit accuracy. As long as the rate of change of the

current is not too fast, an auto-zero amplifier can be used with

excellent results.

AD5541/AD5542

AD8628

DGND

*AD5542 ONLY

VDD REF(REFF*) REFS*

VOUT

SCLK

DIN

AGND

2.5V

UNIPOLAR

OUTPUT

LDAC*

0.1µF

10µF

0.1µF

SERIAL

INTERFACE

Figure 64. AD8628 Used as an Output Amplifier

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