AD647

REV. A

–5–

APPLICATION NOTES

The AD647 is fully specified under actual operating conditions

to insure high performance in any application, but there are some

steps that will improve on even this high level of performance.

The bias current of a JFET amplifier doubles with every 10

°C

increase in junction temperature. Any heat source that can be

eliminated or minimized will significantly improve bias current

performance. To account for normal power dissipation, the larg-

est contributor to chip self-heating, the bias currents of the

AD647 are guaranteed fully warmed up with

±15 V supplies. A

decrease in supply voltage will decrease power consumption,

resulting in a corresponding drop in bias currents.

Open loop gain and bias currents, to some extent, are affected

by output loading. In applications where high linearity is essen-

tial, load impedance should be kept as high as possible to mini-

mize degradation of open loop gain.

The outstanding ac and dc performance of the AD647 make it

an ideal choice for critical instrumentation applications. In such

applications, leakage paths, line losses and external noise

sources should be considered in the layout of printed circuit

boards. A guard ring surrounding the inputs and connected to a

low impedance potential (at the same level as the inputs) should

be placed on both sides of the circuit board. This will eliminate

leakage paths that could degrade bias current performance. All

signal paths should be shielded to minimize noise pickup.

Figure 23. AD647 Used as DAC Output Amplifier

A CMOS DAC AMPLIFIER

The output impedance of a CMOS DAC, such as the AD7541,

varies with digital input code. This causes a corresponding

variation in the noise gain of the DAC-amplifier combination.

This noise gain modulation introduces a nonlinearity whose

magnitude is dependent on the amount of offset voltage present.

Laser wafer drift trimming lowers the initial offset voltage and

the offset voltage drift of the AD647, therefore minimizing the

effect of this nonlinearity and its drift with temperature. This, in

conjunction with the low bias current and high open loop gain,

makes the AD647 ideal for DAC output amplifier applications.

THE AD647 USED WITH THE AD7546

Figure 24 shows the AD647 used with the AD7546 16-bit

segment DAC. In this application, amplifier performance is

critical to the overall performance of the AD7546. A1 is used as

a dual precision buffer. Here the offset voltage match, low offset

voltage and high open loop gain of the AD647 ensure monoto-

nicity and high linearity over the entire operating temperature

range. A2 serves a dual function amplifier A is a Track and

Hold circuit that deglitches the DAC output and amplifier B

acts as an output amplifier. The performance of the amplifiers

of A2 is crucial to the accuracy of the system. The errors of

these amplifiers are added to the errors due strictly to DAC im-

perfections. For this reason great care should be used in the se-

lection of these amplifiers. The matching characteristics, low

bias current and low temperature coefficients of the AD647

make it ideal for this application.

Figure 24. AD647 Used with AD7546 16-Bit DAC

USING THE AD647 IN LOG AMPLIFIER APPLICATIONS

Log amplifiers or log ratio amplifiers are useful in a wide range

of analog computational applications, ranging from the simple

linearization of exponential transducer outputs to the use of

logarithms in computations involving multi-term products or ar-

bitrary exponents. Log amps also facilitate the compression of

wide ranging analog input signals into a range that can be easily

handled using standard circuit techniques.

Figure 25. Log-Ratio Amplifier