Datasheet AMP02 (Analog Devices) - 9

AMP02 APPLICATIONS INFORMATION
The voltage gain can range from 1 to 10,000. A gain set resistor is
Input and Output Offset Voltages
not required for unity-gain applications. Metal-film or wirewound Instrumentation amplifiers have independent offset voltages resistors are recommended for best results. associated with the input and output stages. The input offset The total gain accuracy of the AMP02 is determined by the component is directly multiplied by the amplifier gain, whereas tolerance of the external gain set resistor, RG, combined with the output offset is independent of gain. Therefore at low gain, gain equation accuracy of the AMP02. Total gain drift combines output-offset errors dominate while at high gain, input-offset the mismatch of the external gain set resistor drift with that of the errors dominate. Overall offset voltage, VOS, referred to the internal resistors (20 ppm/°C typ). Maximum gain drift of the output (RTO) is calculated as follows: AMP02 independent of the external gain set resistor is 50 ppm/°C. V RTO V G V ( ) = ( × ) + OS IOS OOS All instrumentation amplifiers require attention to layout so thermocouple effects are minimized. Thermocouples formed where VIOS and VOOS are the input and output offset voltage between copper and dissimilar metals can easily destroy the specifications and G is the amplifier gain. TCVOS performance of the AMP02, which is typically 0.5 µV/°C. The overall offset voltage drift TCVOS, referred to the output, is Resistors themselves can generate thermoelectric EMFs when a combination of input and output drift specifications. Input mounted parallel to a thermal gradient. offset voltage drift is multiplied by the amplifier gain, G, and The AMP02 uses the triple op amp instrumentation amplifier summed with the output offset drift: configuration with the input stage consisting of two transimped- TCV RTO TCV G TCV ance amplifiers followed by a unity-gain differential amplifier. ( ) = ( × ) + OS IOS OOS The input stage and output buffer are laser-trimmed to increase where TCVIOS is the input offset voltage drift, and TCVOOS is gain accuracy. The AMP02 maintains wide bandwidth at all the output offset voltage drift. Frequently, the amplifier drift is gains as shown in Figure 3. For voltage gains greater than 10, referred back to the input (RTI), which is then equivalent to an the bandwidth is over 200 kHz. At unity gain, the bandwidth of input signal change: the AMP02 exceeds 1 MHz. TCVOOS
80
TCV RTI TCV ( ) = + OS IOS
TA = 25
ⴗ
C
G
VS =
ⴞ
15V G = 1000
For example, the maximum input-referred drift of an
60
AMP02EP set to G = 1000 becomes:
G = 100 40
100 o o µV C TCV RTI 2 µV C 2 1 µV C ( ) = + = o . OS 1000
G = 10 20 Input Bias and Offset Currents GE GAIN – dB LTA
Input transistor bias currents are additional error sources that
G = 1 0 VO
can degrade the input signal. Bias currents flowing through the signal source resistance appear as an additional offset voltage.
–20
Equal source resistance on both inputs of an IA will minimize offset changes due to bias current variations with signal voltage
–40
and temperature; however, the difference between the two bias
1k 10k 100k 1M 10M FREQUENCY – Hz
currents (the input offset current) produces an error. The mag- nitude of the error is the offset current times the source resistance. Figure 3. The AMP02 Keeps Its Bandwidth at A current path must always be provided between the differential High Gains inputs and analog ground to ensure correct amplifier operation.
Common-Mode Rejection
Floating inputs such as thermocouples should be grounded Ideally, an instrumentation amplifier responds only to the differ- close to the signal source for best common-mode rejection. ence between the two input signals and rejects common-mode
Gain
voltages and noise. In practice, there is a small change in output The AMP02 only requires a single external resistor to set the voltage when both inputs experience the same common-mode voltage gain. The voltage gain, G, is: voltage change; the ratio of these voltages is called the common-mode gain. Common-mode rejection (CMR) is the G = 50 kΩ +1 logarithm of the ratio of differential-mode gain to common-mode RG gain, expressed in dB. Laser trimming is used to achieve the and high CMR of the AMP02. 50 kΩ RG = G –1 –8– REV. E Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE WAFER TEST LIMITS Typical Performance Characteristics APPLICATIONS INFORMATION Input and Output Offset Voltages Input Bias and Offset Currents Gain Common-Mode Rejection Grounding Sense and Reference Terminals Overvoltage Protection Power Supply Considerations OUTLINE DIMENSIONS Revision History