Datasheet AD549 (Analog Devices) - 16
| Manufacturer | Analog Devices |
| Description | Ultralow Input Bias Current Operational Amplifier |
| Pages / Page | 18 / 16 — AD549. Data Sheet. TEMPERATURE COMPENSATED pH PROBE. AMPLIFIER. FOR EACH … |
| Revision | K |
| File Format / Size | PDF / 1.2 Mb |
| Document Language | English |
AD549. Data Sheet. TEMPERATURE COMPENSATED pH PROBE. AMPLIFIER. FOR EACH AMPLIFIER. 10kΩ. PIN 7. +VS. 0.1µF. OFFSET. PIN 4. –VS. Q1, Q2 = LM394
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Text Version of Document
link to page 17
AD549 Data Sheet
Frequency compensation is provided by R11, R12, C1, and C2. composed of R13, D1, R16, R14, D2, and R17 compensate for The bandwidth of the circuit is 300 kHz at input signals greater these errors, so that this circuit has less than a 1% log confor- than 50 μA; bandwidth decreases smoothly with decreasing mance error at 1 mA input currents. The correct value for R13 signal levels. and R14 depends on the type of log transistors used. The 49.9 kΩ To trim the circuit, set the input currents to 10 μA and trim the resistors were chosen for use with LM394 transistors. Smaller A3 offset using the trim potentiometer of the amplifier for the resistance values are needed for smaller log transistors. output to equal 0. Next, set I1 to 1 μA and adjust the output to
TEMPERATURE COMPENSATED pH PROBE
equal 1 V by trimming R10. Additional offset trims on Ampli-
AMPLIFIER
fier A1 and Amplifier A2 can be used to increase the voltage A pH probe can be modeled as an mV-level voltage source input accuracy and dynamic range. with a series source resistance dependent on the electrode The very low input current of the AD549 makes this circuit composition and configuration. The glass bulb resistance of a useful over a very wide range of signal currents. The total input typical pH electrode pair falls between 106 Ω and 109 Ω. It is current (which determines the low level accuracy of the circuit) therefore important to select an amplifier with low enough is the sum of the amplifier input current, the leakage across the input currents such that the voltage drop produced by the compensating capacitor (negligible if a polystyrene or Teflon amplifier input bias current and the electrode resistance does capacitor is used), and the collector-to-collector and collector- not become an appreciable percentage of a pH unit. to-base leakages of one side of the dual log transistors. The The circuit in Figure 46 illustrates the use of the AD549 as a pH magnitudes of these last two leakages depend on the amplifier probe amplifier. As with other electrometer applications, the use of input offset voltage and are typically less than 10 fA with 1 mV guarding, shielding, and Teflon standoffs is necessary to capitalize offsets. The low level accuracy is limited primarily by the on the AD549 low input current. If an AD549L (60 fA maximum amplifier input current, only 60 fA maximum when the input current) is used, the error contributed by the input current is AD549L is used. held below 60 μV for pH electrode source impedances up to 109 Ω. The effects of the emitter resistance of Q1 and Q2 can degrade Input offset voltages (which can be trimmed) are below 0.5 mV. circuit accuracy at input currents above 100 μA. The networks
FOR EACH AMPLIFIER 10kΩ PIN 7 +VS 0.1µF 4 V1 3 1 OFFSET 0.1µF 5 A1 D3 PIN 4 –VS 6 AD549 Q1, Q2 = LM394 I1 IN 2 DUAL LOG TRANSISTORS R11 R15 C1 4.99kΩ 1kΩ 100pF R1 * 10kΩ A R3 R5 R7 V1 IN Q1 20kΩ 20kΩ 15kΩ R16 10Ω 3 R14 D1 49.9kΩ A3 VOUT 6 AD549 5 D2 SCALE 2 1 R10 FACTOR R17 R13 4 10kΩ 2kΩ ADJ 10Ω 49.9kΩ OUTPUT OFFSET R2 10kΩ R4 R6 R9 V2 IN Q2 20kΩ 20kΩ * R8 14.3kΩ C2 B 1kΩ 100pF V V 2 4.99kΩ OUT = 1V × LOG10 V1 I I 2 2 2 IN VOUT = 1V × LOG10 I A2 D4 1 6 AD549 D1, D4 1N4148 DIODES 5 R8, R15 1kΩ + 350 ppm/°C TC RESISTOR *TELLAB QB1 OR PRECISION RESISTOR PT146 3 1 ALL OTHER RESISTORS ARE 1% METAL FILM 4 10kΩ
45 0
V
1-
2
51
OFFSET
00 Figure 45. Log Ratio Amplifier Rev. K | Page 16 of 18 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS FUNCTIONAL DESCRIPTION MINIMIZING INPUT CURRENT CIRCUIT BOARD NOTES OFFSET NULLING AC RESPONSE WITH HIGH VALUE SOURCE AND FEEDBACK RESISTANCE COMMON-MODE INPUT VOLTAGE OVERLOAD DIFFERENTIAL INPUT VOLTAGE OVERLOAD INPUT PROTECTION SAMPLE-AND-DIFFERENCE CIRCUIT TO MEASURE ELECTROMETER LEAKAGE CURRENTS PHOTODIODE INTERFACE Photodiode Preamp Noise LOG RATIO AMPLIFIER TEMPERATURE COMPENSATED pH PROBE AMPLIFIER OUTLINE DIMENSIONS ORDERING GUIDE
AD549 Data Sheet
Frequency compensation is provided by R11, R12, C1, and C2. composed of R13, D1, R16, R14, D2, and R17 compensate for The bandwidth of the circuit is 300 kHz at input signals greater these errors, so that this circuit has less than a 1% log confor- than 50 μA; bandwidth decreases smoothly with decreasing mance error at 1 mA input currents. The correct value for R13 signal levels. and R14 depends on the type of log transistors used. The 49.9 kΩ To trim the circuit, set the input currents to 10 μA and trim the resistors were chosen for use with LM394 transistors. Smaller A3 offset using the trim potentiometer of the amplifier for the resistance values are needed for smaller log transistors. output to equal 0. Next, set I1 to 1 μA and adjust the output to
TEMPERATURE COMPENSATED pH PROBE
equal 1 V by trimming R10. Additional offset trims on Ampli-
AMPLIFIER
fier A1 and Amplifier A2 can be used to increase the voltage A pH probe can be modeled as an mV-level voltage source input accuracy and dynamic range. with a series source resistance dependent on the electrode The very low input current of the AD549 makes this circuit composition and configuration. The glass bulb resistance of a useful over a very wide range of signal currents. The total input typical pH electrode pair falls between 106 Ω and 109 Ω. It is current (which determines the low level accuracy of the circuit) therefore important to select an amplifier with low enough is the sum of the amplifier input current, the leakage across the input currents such that the voltage drop produced by the compensating capacitor (negligible if a polystyrene or Teflon amplifier input bias current and the electrode resistance does capacitor is used), and the collector-to-collector and collector- not become an appreciable percentage of a pH unit. to-base leakages of one side of the dual log transistors. The The circuit in Figure 46 illustrates the use of the AD549 as a pH magnitudes of these last two leakages depend on the amplifier probe amplifier. As with other electrometer applications, the use of input offset voltage and are typically less than 10 fA with 1 mV guarding, shielding, and Teflon standoffs is necessary to capitalize offsets. The low level accuracy is limited primarily by the on the AD549 low input current. If an AD549L (60 fA maximum amplifier input current, only 60 fA maximum when the input current) is used, the error contributed by the input current is AD549L is used. held below 60 μV for pH electrode source impedances up to 109 Ω. The effects of the emitter resistance of Q1 and Q2 can degrade Input offset voltages (which can be trimmed) are below 0.5 mV. circuit accuracy at input currents above 100 μA. The networks
FOR EACH AMPLIFIER 10kΩ PIN 7 +VS 0.1µF 4 V1 3 1 OFFSET 0.1µF 5 A1 D3 PIN 4 –VS 6 AD549 Q1, Q2 = LM394 I1 IN 2 DUAL LOG TRANSISTORS R11 R15 C1 4.99kΩ 1kΩ 100pF R1 * 10kΩ A R3 R5 R7 V1 IN Q1 20kΩ 20kΩ 15kΩ R16 10Ω 3 R14 D1 49.9kΩ A3 VOUT 6 AD549 5 D2 SCALE 2 1 R10 FACTOR R17 R13 4 10kΩ 2kΩ ADJ 10Ω 49.9kΩ OUTPUT OFFSET R2 10kΩ R4 R6 R9 V2 IN Q2 20kΩ 20kΩ * R8 14.3kΩ C2 B 1kΩ 100pF V V 2 4.99kΩ OUT = 1V × LOG10 V1 I I 2 2 2 IN VOUT = 1V × LOG10 I A2 D4 1 6 AD549 D1, D4 1N4148 DIODES 5 R8, R15 1kΩ + 350 ppm/°C TC RESISTOR *TELLAB QB1 OR PRECISION RESISTOR PT146 3 1 ALL OTHER RESISTORS ARE 1% METAL FILM 4 10kΩ
45 0
V
1-
2
51
OFFSET
00 Figure 45. Log Ratio Amplifier Rev. K | Page 16 of 18 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS FUNCTIONAL DESCRIPTION MINIMIZING INPUT CURRENT CIRCUIT BOARD NOTES OFFSET NULLING AC RESPONSE WITH HIGH VALUE SOURCE AND FEEDBACK RESISTANCE COMMON-MODE INPUT VOLTAGE OVERLOAD DIFFERENTIAL INPUT VOLTAGE OVERLOAD INPUT PROTECTION SAMPLE-AND-DIFFERENCE CIRCUIT TO MEASURE ELECTROMETER LEAKAGE CURRENTS PHOTODIODE INTERFACE Photodiode Preamp Noise LOG RATIO AMPLIFIER TEMPERATURE COMPENSATED pH PROBE AMPLIFIER OUTLINE DIMENSIONS ORDERING GUIDE