feature

Low Offset Voltage: 100 mV max; Low Drift: 2 mV/8C max; Wide Gain Range 1 to 10000; High Common Mode Rejection: 115db min; High Bandwidth (G=1000): 200 kHz Typical; Gain Equation Accuracy: 0.5% max; single resistor gain setting; input overvoltage protection; low cost available in die form.

application

Differential amplifiers; strain gauge amplifiers; thermocouple amplifiers; resistance temperature detector amplifiers; programmable gain instrumentation amplifiers; medical devices; data acquisition systems.

General Instructions

The AMP02 is the first precision instrumentation amplifier available in an 8-pin package. The gain of the AMP02 is determined by a single external resistor, ranging from 1 to 10000. Gain setting resistors are not required for unity gain. The AMP02 includes an input protection network that allows the input to exceed 60V from either rail without damaging the device. Laser trimming reduces the input bias voltage to below 100µV. The output bias voltage is lower than 4mV, and the gain accuracy is better than 0.5% to obtain 1000 points. PMI's proprietary thin film resistor process keeps the gain temperature coefficient below 50ppm/°C. Due to the design of the AMP02, its bandwidth remains high over a wide gain range. Slew rates in excess of 4 V/µs make the AMP02 ideal for fast data acquisition systems.

A reference pin is provided to allow the reference output to an external DC level. This pin can be used for offset correction or horizontal movement as required. In the 8-pin package, the sense is internally connected to the output.

For the most accurate instrumentation amplifiers, refer to the AMP01 datasheet. For maximum input impedance and speed, refer to the AMP5 datasheet.

application information

Input and output offset voltage

Instrumentation amplifiers have independent bias voltages associated with the input and output stages. The input offset component is directly multiplied by the amplifier gain, while the output offset is gain independent. Therefore, at low gains, the output offset error dominates, and at high gains, the input offset error dominates. The total offset voltage VOS (referred to as output (RTO)) is calculated as:

where VIOS and VOOS are the input and output offset voltage specifications, and G is the amplifier gain.

The total offset voltage drift TCVOS, called the output, is the combination of the input and output drift specifications. The input bias voltage drift is multiplied by the amplifier gain G and added to the output bias drift:

where TCVIOS is the input offset voltage drift and TCVOOS is the output offset voltage drift. Typically, amplifier drift is referred back to the input (RTI) and is then equivalent to the change in the input signal:

For example, the maximum input reference drift of an AMP02EP set to g=1000 becomes:

Input Bias and Bias Current

The input transistor bias current is an additional source of error that can degrade the input signal. The bias current flowing through the signal source resistor is shown as an additional bias voltage. Equal source resistance on both inputs of the IA minimizes offset changes due to changes in bias current with signal voltage and temperature. However, the difference between the two bias currents (input bias currents) creates errors. The magnitude of the error is the offset current times the source resistance.

A current path must always be provided between the differential input and analog ground to ensure proper operation of the amplifier. Floating inputs such as thermocouples should be grounded close to the signal source for best common-mode rejection.

get

The AMP02 only needs an external resistor to set the voltage gain. The voltage gain G is:

and

The voltage gain can be from 1 to 10000. Unity gain applications do not require gain setting resistors. It is best to use metal film or wirewound resistors.

The overall gain accuracy of the AMP02 is determined by the tolerance of the external gain setting resistor RG and the accuracy of the gain equation of the AMP02. The total gain drift combines the mismatch between the external gain setting resistor drift and the internal resistor drift (20 ppm/°C typical). The AMP02 has a maximum gain drift of 50 ppm/°C independent of the external gain setting resistors.

All instrumentation amplifiers require careful layout to minimize thermocouple effects. Thermocouples formed between copper and dissimilar metals are easily destroyed

Typical TCVOS performance of AMP02

The 0.5 microvolts/degree Celsius resistor itself generates a thermoelectromotive force when mounted parallel to a thermal gradient.

The AMP02 uses a three-op amp instrumentation amplifier configuration with an input stage consisting of two transimpedance amplifiers and a unity-gain differential amplifier. The input stage and output buffer are laser trimmed to improve gain accuracy. As shown in Figure 26, the AMP02 remains wideband at all gains. For voltage gains greater than 10, the bandwidth exceeds 200khz. At unity gain, the bandwidth of the AMP02 exceeds 1 MHz.

Common Mode Rejection

Ideally, an instrumentation amplifier responds only to the difference between the two input signals and rejects common-mode voltages and noise. In practice, when both inputs experience the same common-mode voltage change, there is a small change in the output voltage; the ratio of these voltages is called the common-mode gain. Common Mode Rejection (CMR) is the logarithm of the ratio of differential mode gain to common mode gain in decibels. High CMR of AMP02 is achieved with laser trimming.

Figure 27 shows the triple op amp configuration for the AMP02. With all instrumentation amplifiers of this type, it is critical not to exceed the dynamic range of the input amplifier. The amplified differential input signal and input common-mode voltage must not force the amplifier's output voltage to exceed ±12 V (V s = ±15 V), otherwise non-linear operation will result.

The output voltage of the input stage amplifier at V and V2 is equal to:

Where: VD=differential input voltage=(+IN)–(–IN); VCM=common-mode input voltage; G=instrumentation amplifier gain; if the maximum value of V1 and V2 is equal to 12 V, then the common-mode input voltage range is:

ground

Most instrumentation and data acquisition systems have separate grounds for analog and digital signals. Analog grounds can also be split into two or more grounds that will be connected together at one point, usually the analog power ground. Additionally, digital and analog grounds can be connected, usually at the analog ground pin on the A-to-D converter. Following this basic practice is critical to good circuit performance.

Mixed fields cause interactions between digital circuits and analog signals. Because ground loops have limited resistance and inductance, hundreds of millivolts can develop between system ground and data acquisition components.

Use a separate ground return to minimize current flow from sensitive analog loops to system ground. Therefore, the noisy ground current from the logic gate does correlate with the analog signal.

Inevitably, two or more circuits will be connected together with their ground at a differential potential. In these cases, the differential input of the instrumentation amplifier, with its high CMR, can accurately transfer analog information from one circuit to another.

Sense and Reference Terminals

The sense terminal completes the feedback path for the in-amp output stage and is internally connected directly to the output. For SOL devices, connect the detection terminal to the output. The output signal is specified relative to a reference terminal normally connected to analog ground. This reference can also be used for offset correction level shifts. The reference source resistor will reduce the CMRR to 25 kΩ/RREF. If the reference source resistance is 1Ω, the CMR will be reduced by 88 dB (25 kΩ/1Ω = 88 dB).

Over voltage protection

Instrumentation amplifiers are always located at the front end of the instrumentation system, where there is a high chance of being exposed to overload. When a signal source is connected, voltage transients, sensor failure, or removal of amplifier power can damage or degrade the performance of unprotected equipment. A common technique is to place limiting resistors in series with each input, but this adds noise. The AMP02 includes internal protection circuitry that limits the input current to ±4 mA for a 60 V differential overload (see Figure 28) in the power-off condition and ±2.5 mA in the powered-up condition.

Current under overvoltage condition

Power Considerations

In practical circuits, achieving the rated performance of precision amplifiers requires careful attention to external influences. For example, power supply noise and variations in nominal voltage directly affect the input bias voltage. A PSR of 80dB means that a change of 100mV on the power supply (not an uncommon value) will produce a change in input offset of 10µV. Therefore, care should be taken when selecting a power unit with low output noise levels, good line and load regulation, and good temperature stability. Additionally, each power supply should be properly bypassed.

Dimensions

Dimensions are in inches and (mm).