AD647

REV. A

–6–

The picoamp level input current and low offset voltage of the

AD647 make it suitable for wide dynamic range log amplifiers.

Figure 25 is a schematic of a log ratio circuit employing the

AD647 that can achieve less than 1% conformance error over

5 decades of current input, 1 nA to 100

µA. For voltage inputs,

the dynamic range is typically 50 mV to 10 V for 1% error,

limited on the low end by the amplifiers’ input offset voltage.

The conversion between current (or voltage) input and log out-

put is accomplished by the base-emitter junctions of the dual

transistor Q1. Assuming Q1 has

β > 100, which is the case for

the specified transistor, the base-emitter voltage on side 1 is to a

close approximation

and Q1B, thus producing an output voltage proportional to the

log of the ratio of the inputs

KkT

q

produce a 1 V change in output voltage per decade difference in

°C

temperature coefficient, which makes K inversely proportional

to temperature, compensating for the “T” in kT/q. The log ratio

transfer characteristic is therefore independent of temperature.

This particular log ratio circuit is free from the dynamic prob-

lems that plague many other log circuits. The –3 dB bandwidth

is 50 kHz over the top 3 decades, 100 nA to 100

µA, and de-

creases smoothly at lower input levels. This circuit needs no

additional frequency compensation for stable operation from

input current sources, such as photodiodes, which may have

100 pF of shunt capacitance. For larger input capacitances a

20 pF integration capacitor around each amplifier will provide a

smoother frequency response.

This log ratio amplifier can be readily adjusted for optimum

accuracy by following this simple procedure. First, apply V1 =

+1.00 V. Repeat this procedure until gain and balance readings

are within 2 mV of ideal values.

ACTIVE FILTERS

In active low-pass filtering applications the dc accuracy of the

amplifiers used is critical to the performance of the filter cir-

cuits. DC error sources such as offset voltage and bias currents

represent the largest individual contributors to output error.

Offset voltages will be passed by the filtering network and may,

depending on the design of the filter circuit, be amplified and

generate unacceptable output offset voltages. In filter circuits for

low frequency ranges large value resistors are used to generate

the low-pass filter function. Input bias currents passing through

these resistors will generate an additional offset voltage that will

also be passed to the output of the filter.

The use of the AD647 will minimize these error sources and,

therefore, maximize filter accuracy. The wide variety of perfor-

mance levels of the AD647 allows for just the amount of accu-

racy required for any given application.

AD647 AS AN INSTRUMENTATION AMPLIFIER

The circuit shown in Figure 26 uses the AD647 to construct an

ultra high precision instrumentation amplifier. In this type of

application the matching characteristics of a monolithic dual

amplifier are crucial to ensure high performance.

Figure 26. Precision FET Input Instrumentation Amplifier

The use of an AD647L as the input amplifier A1, guarantees

maximum offset voltage of 250

µV, drift of 2.5 µV/°C and bias

currents of 35 pA. A2 serves two less critical functions in the

amplifier and, therefore can be an AD647J. Amplifier A is an ac-

tive data guard which increases ac CMRR and minimizes extra-

neous signal pickup and leakage. Amplifier B is the output

amplifier of the instrumentation amplifier. To attain the preci-

sion available from this configuration, a great deal of care

should be taken when selecting the external components.

CMRR will depend on the matching of resistors R1, R2, R3,

and R4. The gain drift performance of this circuit will be af-

fected by the matching TC of the resistors used.

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-99

E-20A