AD8638/AD8639

Rev. C | Page 15 of 20

INFRARED SENSORS

Infrared (IR) sensors, particularly thermopiles, are increasingly

used in temperature measurement for applications as wide

ranging as automotive climate control, human ear thermometers,

home insulation analysis, and automotive repair diagnostics.

The relatively small output signal of the sensor demands high

gain with very low offset voltage and drift to avoid dc errors.

If interstage ac coupling is used, as shown in Figure 52, low

offset and drift prevent the output of the input amplifier from

drifting close to saturation. The low input bias currents generate

minimal errors from the output impedance of the sensor.

Similar to pressure sensors, the very low amplifier drift with

time and temperature eliminates additional errors once the

system is calibrated at room temperature. The low 1/f noise

improves SNR for dc measurements taken over periods often

exceeding one-fifth of a second.

Figure 52 shows a circuit that can amplify ac signals from

100 μV to 300 μV up to the 1 V to 3 V levels, with a gain of

10,000 for accurate analog-to-digital conversions.

5V TO 16V

100kΩ

10kΩ

5V TO 16V

100µV TO 300µV

100Ω

TO BIAS

VOLTAGE

10kΩ

IR

DETECTOR

100kΩ

10µF

1/2 AD8639

1/2 AD8639

Figure 52. AD8639 Used as a Preamplifier for Thermopile

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 53. 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.

0.1Ω

SUPPLY

I

100Ω

100kΩ

C

5V TO 16V

100Ω

100kΩ

C

100mV/mA

AD8638

Figure 53. 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 may 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 to prevent injected bias current

from becoming a significant percentage of the measured current.

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

overall circuit accuracy. With the extremely high CMRR of the

AD8638/AD8639, the CMRR is limited by the resistor ratio

matching. 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 AD8638/AD8639 can be 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 operating 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 offset error when multiplied

by the DAC output impedance (approximately 6 kΩ).

Rail-to-rail output provides full-scale output with very little

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

independent, but the high input impedance of the AD8638/

AD8639 minimizes gain errors. The wide bandwidth of the

amplifier also serves well in this case. The amplifier, with a

settling time of 4 μs, adds another time constant to the system,

increasing the settling time of the output. For example, see

Figure 54. The settling time of the AD5541 is 1 μs. The

combined settling time is approximately 4.1 μs, as can be

derived from the following equation:

()

(

2

8638

AD

t

DAC

t

TOTAL

t

S

S

S

+

=

AD5541/AD5542

ADR421

AD8638

DGND

*AD5542 ONLY

REF(REFF*) REFS*

SCLK

DIN

CS

AGND

5V

UNIPOLAR

OUTPUT

LDAC*

0.1µF

0.1µF

2.5V

62

4

0.1µF

SERIAL

INTERFACE

5V TO 16V

5V TO 16V

Figure 54. AD8638 Used as an Output Amplifier