feature

Ease of use; gain setting with one external resistor (gain range 1 to 1000 ); wide supply range (62.3 V to 618 V); higher performance than three op amp designs; available in 8-lead dipping and SOIC packages; low power, 1.3 mA maximum supply current; excellent DC performance ("Class B"); 50 mV maximum, input offset voltage; 0.6 mV/8C maximum, input offset drift; 1.0 mA maximum, input bias current; 100dB minimum Common Mode Rejection Ratio (G=10); Low Noise; 9 nV/√Hz, @1 kHz, Input Voltage Noise; 0.28 mV pp Noise (0.1 Hz to 10 Hz); Excellent AC Specifications; 120 kHz Bandwidth (G= 100); 15ms precipitation time to 0.01%.

application

Weighing scales; ECG and medical devices; sensor interfaces; data acquisition systems; industrial process controls; battery powered and portable devices.

Product Description

The AD620 is a low-cost, high-precision instrumentation amplifier that requires only one external resistor to set the gain from 1 to 1000. In addition, the AD620 features an 8-lead SOIC and DIP in a smaller package than standalone designs, provides lower power (only 1.3 mA maximum supply current), and is ideal for battery-powered, portable (or remote) applications.

The AD620, with high precision nonlinearity up to 40 ppm, low bias voltage of 50 μV maximum and 0.6 μV/C maximum, is ideal for precision data acquisition systems such as weighing scales and sensor interfaces. Low noise, low input bias current, and low power consumption make the AD620 ideal for medical applications as an electrocardiogram and non-invasive blood pressure monitor.

A low input bias current of 1.0 mA maximum is possible at the input stage using hyper-beta eta processing. Due to its low input voltage noise, the AD620 is 9 nV/√Hz at 1 kHz, 0.28 μV pp in the 0.1 Hz to 10 Hz frequency band, and 0.1 pa/√Hz input current noise. Also, the AD620 is well suited for multiplexing applications, with settling times of 15 microseconds to 0.01%, and its low cost enough to enable designs with 1 inch amp per channel.

metallized photo

Dimensions are in inches and (mm).

Contact factory for latest sizes.

be careful

Electrostatic discharge sensitive devices. Electrostatic charges of up to 4000 volts build up on the human body and test equipment and can discharge without detection. warn!

Although the AD620 has proprietary ESD protection circuitry, permanent damage to devices exposed to high-energy electrostatic discharges may occur. Therefore, proper ESD electrostatic sensitive devices recommend taking precautions to avoid performance degradation or loss of functionality.

Typical Characteristics (@+258C, VS=615V, RL=2kV, unless otherwise stated)

theory of operation

The AD620 is a monolithic instrumentation amplifier based on a modification of the classic three-op-amp approach. Absolute trimming allows the user to precisely program the gain (0.15% at G=100) using only one resistor. Monolithic construction and laser wafer trimming allow for tight matching and tracking of circuit elements, ensuring the high level of performance inherent in the circuit.

Input transistors Q1 and Q2 provide a high-accuracy single-differential-pair bipolar input (Figure 33), but provide 10 times lower input bias current due to super-beta processing. Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintains a constant collector current of the input devices Q1, Q2, thereby affecting the input voltage through the external gain setting resistor RG. This will produce a differential gain from the input to the output of A1/A2 given by G=(R1+R2)/RG+1. Unity-gain subtractor A3 removes any common-mode signal, producing a single-ended output referenced to the pin potential.

The value of RG also determines the transconductance of the preamplifier stage. When RG is decreased for greater gain, the transconductance and the transconductance of the input transistor gradually increase. This has three important advantages: (a) Increases the programmed gain by increasing the open-loop gain, thereby reducing gain-dependent errors. (b) The gain-bandwidth product (determined by C1, C2, and the preamplifier transconductance) increases with the programmed gain to optimize the frequency response. (c) The input voltage noise is reduced to 9nv/√, mainly determined by the collector current and base resistance of the input device.

Internal gain resistors R1 and R2 are adjusted to an absolute value of 24.7kΩ, allowing precise programming of gain using a single external resistor.

The gain equation is:

Make vs. Buy: Typical Bridging Application Error Budget Compared to "Homemade" Three Op Amp IA Designs, AD620 Offers Better Performance While Having Smaller Size, Fewer Components, and 10x Lower Supply current. In a typical application, as shown in Figure 34, a gain of 100 is required to amplify a bridge output of 20 mV full-scale over the industrial temperature range of -40°C to +85°C. The error budget table below shows how to calculate the effect of various error sources on circuit accuracy.

Regardless of the system used, the AD620 provides higher accuracy at lower power consumption and at a price. In simple systems, absolute accuracy and drift errors are by far the most important sources of error. In more complex systems with intelligent processors, the self-gain/self-zero period will remove all absolute accuracy and drift errors, leaving only the resolution errors of gain nonlinearity and noise, allowing full 14-bit accuracy.

Note that for the homebrew circuit, the OP07 specifications for input voltage offset and noise are multiplied by √. This is because a three-op amp type amplifier has two op amps at its input, both of which contribute to the overall input error.

pressure measurement

While the AD620 is useful in many bridge applications such as weighing scales, it is particularly useful for high resistance pressure sensors powered at low voltages where small size and low power become more important.

Figure 35 shows a 3 kΩ pressure sensor bridge powered by +5 V. In this circuit, the bridge consumes only 1.7 mA. With the addition of the AD620 and the buffered voltage divider, the signal only needs to regulate a total supply current of 3.8 mA.

Small size and low cost make the AD620 particularly attractive for voltage output pressure sensors. As it offers low noise and drift, it will also serve applications such as diagnostic non-invasive blood pressure measurement.

Medical ECG

The low current noise of the AD620 allows its use in ECG monitors (Figure 36), where source resistances of 1 MΩ or higher are not uncommon. The AD620's low power consumption, low supply voltage requirements, and space-saving 8-lead miniature DIP and SOIC packages make it an excellent choice for battery-operated data loggers.

In addition, the low bias current and low current noise combined with the low voltage noise of the AD620 improve dynamic range for better performance.

The value of capacitor C1 is chosen to maintain the stability of the right leg drive loop. Appropriate protective measures, such as isolation, must be added to the circuit to protect the patient from possible harm.

Precision VI Converter

Together with another op amp and two resistors, the AD620 forms an accurate current source (Figure 37). The op amp buffers the reference terminal to maintain good CMR. The AD620's output voltage, VX, appears on R1, which converts it to current. The smaller this current is, the input bias current of the op amp, which then flows to the load.

Gain selection

The gain of the AD620 is a resistor programmed by RG or, more precisely, through whatever impedance appears between pins 1 and 8. The AD620 is designed to provide accurate gain using 0.1%-1% resistors. Table II shows the RG values required for various gains. Note that for G=1, the RG pin is unconnected (RG=∞). For any gain, RG can be calculated using the following formula:

To reduce gain error, avoid high parasitic resistance in series with RG; to reduce gain drift, the low TC of RG should be less than 10ppm/°C - for best performance.

Input and output bias voltage

The low error of the AD620 is due to two sources, input and output error. When referring to the input, the output error is divided by G. In practical applications, the input error dominates at high gain and the output error dominates at low gain. The total VOS for a given gain is calculated as:

Total Error RTI = input error + (output error/G)

Total Error RTO = (input error × G) + output error

Reference terminal

The reference terminal potential defines zero output voltage and is especially useful when the load does not share a precise ground with the rest of the system. It provides a straightforward method to inject a precise offset into the output within the range allowed by the supply voltage. Parasitic resistance should be kept to a minimum for optimum CMR.

input protection

The AD620 has a 400Ω series thin-film resistor at its input and can safely withstand input overloads up to ±15 V or ±60 mA for hours. This is true for all gain and power on/off, which is especially important since the source and amplifier can be powered separately. For a long time, the current should not exceed 6 mA (IIN≤VIN/400Ω). For input overloads other than the power supply, clamping the input to the power supply (using a low leakage diode such as the FD333) will reduce the required resistance, resulting in lower noise.

radio frequency interference

All instrumentation amplifiers can correct for out-of-band signals, and when amplifying small signals, these rectified voltages act as small DC offset errors. The AD620 allows direct access to the input transistor base and transmitter, enabling the user to apply some first-order filtering to unwanted RF signals (Figure 38), where RC < 1/(2πf), where f ≥ the bandwidth of the AD620; C ≤ 150 pF. Matching external capacitors at pins 1 and 8 and pins 2 and 3 helps keep the CMR high.

Common Mode Rejection

Instrumentation amplifiers like the AD620 offer high CMR, which is a measure of the change in output voltage when the two inputs change by an equal amount. These specifications are usually given for full range input voltage variation and specified power supply imbalance.

For best CMR, the reference terminal should be connected to a low impedance point and the difference in capacitance and resistance between the two inputs should be kept to a minimum. In many applications, shielded cables are used to minimize noise, and for optimal CMR overfrequency, the shield should be properly driven. Figures 39 and 40 show an active data protection device configured to improve AC common-mode rejection by "bootstrapping" the capacitance of the input cable shield to minimize capacitance mismatch between inputs.

ground

Since the AD620's output voltage is developed relative to the potential on the reference terminal, it can solve many grounding problems by simply connecting the reference pin to an appropriate "local ground".

To isolate low-level analog signals from the noisy digital environment, many data acquisition parts have separate analog and digital ground pins (Figure 41). It is convenient to use a ground wire; however, the current through the ground wire and the PC operation of the circuit card can cause errors of hundreds of millivolts. Therefore, a separate ground return should be provided to minimize current flow from sensitive points to system ground. These ground returns must be connected together at some point, usually preferably at the ADC package shown.

Ground return for input bias current

The input bias current is the current necessary to bias the input transistors of the amplifier. These currents must have a direct return path; therefore, when amplifying "floating" inputs as shown in Figure 42, a power supply (such as a transformer or AC-coupled power supply) must have a DC path from each input to ground. For more information on amplifier internal applications, see the Instrumentation Amplifier Application Guide (excluding analog devices).

Dimensions

Dimensions are in inches and (mm).