AD8628ART-R2 datasheet(19/24 Pages) AD

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AD8628/AD8629/AD8630

Rev. E | Page 19 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

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 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. 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).

Figure 65. 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.

0.1

SUPPLY

100

100k

100

100k

e = 1,000 RS I

100mV/mA

AD8628

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

little error. Output impedance of the DAC is constant and code-

independent, but the high input impedance of the AD8628/

AD8629/AD8630 minimizes gain errors. The amplifiers’ wide

bandwidth 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 approxi-

mately 1.4 μs, as can be derived from the following equation:

()

(

) ()2

8628

DAC

TOTAL

Figure 65. 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 shunt’s output signal is

hundreds of mV, or even V, 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(REF*)

REFS*

OUT

SCLK

DIN

AGND

2.5V

UNIPOLAR

OUTPUT

LDAC*

0.1

μF

0.1

μF

SERIAL

INTERFACE

Figure 66. AD8628 Used as an Output Amplifier

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