AD542/AD544/AD547

REV. B

–9–

The oscilloscope photo of Figure 31b shows the output of the

circuit of Figure 31a. The upper trace represents the reference

input, and the bottom trace shows the output voltage for a

digital input of all ones on the DAC (Gain 1–2

capacitor across the feedback resistor compensates for the DAC

output capacitance, and the 150 pF load capacitor serves to

minimize output glitches.

Figure 31b. Voltage Output DAC Settling Characteristic

Figure 32a illustrates the 10-bit digital-to-analog converter,

AD7533, connected for bipolar operation. Since the digital

analog input, the circuit can perform a 4-quadrant multiplying

function.

Figure 32a. AD544 Used as DAC Output Amplifiers

32b is the large signal response and Figure 32c is the small sig-

nal response. C1 phase compensation (15 pF) is required for

stability when using high speed amplifiers. C1 is used to cancel

the pole formed by the DAC internal feedback resistance and

the output capacitance of the DAC.

USING THE AD547 IN LOG AMPLIFIER APPLICATIONS

Log amplifiers or log ratio amplifiers are useful in applications

requiring compression of wide-range analog input data, linear-

ization of transducers having exponential outputs, and analog

computing, ranging from simple translation of natural relation-

ships in log form (e.g., computing absorbance as the log-ratio of

input currents), to the use of logarithms in facilitating analog

computation of terms involving arbitrary exponents and

multi-term products and ratios.

The picoamp level input current and low offset voltage of the

AD547 make it suitable for wide dynamic range log amplifiers.

Figure 33 is a schematic of a log ratio circuit employing the

AD547 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.

Figure 33. Log-Ratio Amplifier

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:

Q1A and Q1B, thus producing an output voltage proportional

to the log of the ratio of the inputs:

V

KkT

q

(ln I

V

produce 1 V change in output voltage per decade difference in

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.

Figure 32b. Large Signal

Response

Figure 32c. Small Signal

Response