AD547 Analog Devices, AD547 Datasheet - Page 9

• Manufacturer Part Number: AD547
• Description: High Performance/ BiFET Operational Amplifiers
• Manufacturer: Analog Devices

REV. B
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 32a illustrates the 10-bit digital-to-analog converter,
AD7533, connected for bipolar operation. Since the digital
input can accept bipolar numbers and V
analog input, the circuit can perform a 4-quadrant multiplying
function.
The photos exhibit the response to a step input at V
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.
Figure 32b. Large Signal
Response
Figure 31b. Voltage Output DAC Settling Characteristic
Figure 32a. AD544 Used as DAC Output Amplifiers
Figure 32c. Small Signal
Response
REF
can accept a bipolar
–n
). The 47 pF
REF
. Figure
–9–
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 A. 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
the specified transistor, the base-emitter voltage on side 1 is to a
close approximation:
This circuit is arranged to take the difference of the V
Q1A and Q1B, thus producing an output voltage proportional
to the log of the ratio of the inputs:
The scaling constant, K is set by R1 and R
produce 1 V change in output voltage per decade difference in
input signals. R
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.
V
OUT
– K (V
Figure 33. Log-Ratio Amplifier
TC
BE A
V
is a special resistor with a +3500 ppm/ C
OUT
V
– V
BE A
BE B
AD542/AD544/AD547
K kT /q ln I
)
kT/q ln I
–
KkT
100, which is the case for
q
1
/I
1
(ln I
/ I
S1
TC
2
1
to about 16, to
/I
S1
– ln I
BE
’s of
2
/I
S2
)