ad549sh-883b Analog Devices, Inc., ad549sh-883b Datasheet - Page 16

• Manufacturer Part Number: ad549sh-883b
• Description: Ultralow Input Bias Current Operational Amplifier
• Manufacturer: Analog Devices, Inc.

AD549
Frequency compensation is provided by R11, R12, C1, and C2.
The bandwidth of the circuit is 300 kHz at input signals greater
than 50 μA; bandwidth decreases smoothly with decreasing
signal levels.
To trim the circuit, set the input currents to 10 μA and trim A3’s
offset using the amplifier’s trim potentiometer so the output
equals 0. Then set I
by trimming R10. Additional offset trims on Amplifier A1 and
Amplifier A2 can be used to increase the voltage input accuracy
and dynamic range.
The very low input current of the AD549 makes this circuit
useful over a very wide range of signal currents. The total input
current (which determines the low level accuracy of the circuit)
is the sum of the amplifier input current, the leakage across the
compensating capacitor (negligible if a polystyrene or Teflon
capacitor is used), and the collector-to-collector and collector-
to-base leakages of one side of the dual log transistors. The
magnitudes of these last two leakages depend on the amplifier
input offset voltage, and are typically less than 10 fA with 1 mV
offsets. The low level accuracy is limited primarily by the
amplifier input current, only 60 fA maximum when the
AD549L is used.
The effects of the emitter resistance of Q1 and Q2 can degrade
the circuit’s accuracy at input currents above 100 μA. The net-
works composed of R13, D1, R16, R14, D2, and R17
1
to 1 μA and adjust the output to equal 1 V
V
V
I
I
1
1
2
2
IN
IN
IN
IN
10kΩ
10kΩ
R1
R2
49.9kΩ
3
2
2
3
R14
100pF
100pF
D2
AD549
AD549
C1
C2
4
4
A1
A2
1
Q1
Q2
1
R16
10Ω
R17
10Ω
10kΩ
A
B
5
5
V
OFFSET
10kΩ
V
OFFSET
2
Figure 45. Log Ratio Amplifier
1
6
6
Rev. G | Page 16 of 20
R13
49.9kΩ
D1
20kΩ
R11
4.99kΩ
4.99kΩ
R3
20kΩ
D3
D4
FOR EACH AMPLIFIER
R4
D1, D4 1N4148 DIODES
R8, R15 1kΩ + 350 ppm/°C TC RESISTOR
*
ALL OTHER RESISTORS ARE 1% METAL FILM
TELLAB QB1 OR PRECISION RESISTOR PT146
PIN 7
PIN 4
Q1, Q2 = LM394
DUAL LOG TRANSISTORS
3
2
20kΩ
compensate for these errors, so that this circuit has less than a
1% log
conformance error at 1 mA input currents. The correct value
for R13 and R14 depends on the type of log transistors used.
The 49.9 kΩ resistors were chosen for use with LM394 transis-
tors. Smaller resistance values are needed for smaller log
transistors.
TEMPERATURE COMPENSATED pH PROBE
AMPLIFIER
A pH probe can be modeled as a mV-level voltage source
with a series source resistance dependent on the electrode’s
composition and configuration. The glass bulb resistance of a
typical pH electrode pair falls between 10
therefore important to select an amplifier with low enough
input currents such that the voltage drop produced by the
amplifier’s input bias current and the electrode resistance does
not become an appreciable percentage of a pH unit.
The circuit in Figure 46 illustrates the use of the AD549 as a pH
probe amplifier. As with other electrometer applications, the use
of guarding, shielding, and Teflon standoffs is necessary to
capitalize on the AD549 low input current. If an AD549L (60 fA
max input current) is used, the error contributed by the input
current is held below 60 μV for pH electrode source impedances
up to 10
below 0.5 mV.
20kΩ
AD549
R6
R5
4
A3
1
*
R8
1kΩ
9
5
OUTPUT
OFFSET
10kΩ
Ω. Input offset voltages (which can be trimmed) are
15kΩ
R15
1kΩ
R7
6
0.1µF
0.1µF
14.3kΩ
V
V
OUT
OUT
*
R9
2kΩ
R10
= 1V × LOG
= 1V × LOG
+V
–V
S
V
S
OUT
SCALE
FACTOR
ADJ
10
10
V
V
I
I
2
1
2
1
6
Ω and 10
9
Ω. It is