AD8628/AD8629

Rev. C | Page 18 of 20

PRECISION CURRENT SHUNTS

A precision shunt current sensor benefits from the unique

attributes of auto-zero amplifiers when used in a differencing

configuration (Figure 63). Shunt current 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

Ω

SUPPLY

I

100

Ω

100k

Ω

C

5V

100

Ω

100k

Ω

C

100mV/mA

AD8628

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

without 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 all 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.

OUTPUT AMPLIFIER FOR HIGH PRECISION DACs

The AD8628/AD8629 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 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 approximately 1.4 µs, as can

be derived from the following equation:

()

(

2

8628

AD

t

DAC

t

TOTAL

t

S

S

S

+

=

AD5541/AD5542

AD8628

DGND

*AD5542 ONLY

REF(REF*)

REFS*

OUT

SCLK

DIN

CS

AGND

5V

2.5V

UNIPOLAR

OUTPUT

LDAC*

0.1

10

0.1

SERIAL

INTERFACE

Figure 64. AD8628 Used as an Output Amplifier