AD8628ART-REEL7 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD8628ART-REEL7

Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifier

Analog Devices 

PRECISION CURRENT SHUNT SENSOR

A precision current shunt sensor benefits from the unique

attributes of auto-zero amplifiers when used in a differencing

configuration, as shown in 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.

SUPPLY

RS

I 0.1Ω

RL

100kΩ

100Ω

e = 1,000 RS I

100mV/mA

C

5V

AD8628

100kΩ

100Ω

C

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.

AD8628/AD8629/AD8630

OUTPUT AMPLIFIER FOR HIGH PRECISION DACS

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).

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:

( ) tS (TOTAL) = tS DAC 2 + (tS AD8628)2

5V

0.1μF

2.5V 10μF

0.1μF

SERIAL

INTERFACE

VDD

CS

DIN

SCLK

REF(REF*) REFS*

AD5541/AD5542

OUT

LDAC*

DGND

AGND

*AD5542 ONLY

UNIPOLAR

OUTPUT

AD8628

Figure 66. AD8628 Used as an Output Amplifier

Rev. E | Page 19 of 24

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