feature

Ease of use; space-saving MSOP available; gain setting with 1 external resistor (gain range 1 to 1000 ) Wide supply range: ±2.3 V to ±8 V Temperature range for specified performance: -40°C to + 85°C; operating temperature up to 125°C[one]; excellent AC specifications; min CMRR 80dB to 10kHz (G=1); 825 kHz, -3 dB bandwidth (G=1); 2 Volts/µs slew rate; low noise; 8 nV/z Hz, @ 1 kHz, maximum input voltage noise; 0.25 μV pp input noise (0.1 Hz to 10 Hz); high precision DC performance ( AD8221BR ); minimum common mode rejection Ratio 90dB (G=1); 25V maximum input offset voltage; 0.3V/°C maximum input offset drift; 0.4 Na maximum input bias current.

application

Scales; Industrial Process Control Bridge Amplifiers; Precision Data Acquisition Systems; Medical Devices; Strain Gauges; Sensor Interfaces.

General Instructions

The AD8221 is a gain programmable, high performance instrumentation amplifier that provides the industry's highest common-mode rejection ratio overfrequency. The common-mode rejection ratio of instrumentation amplifiers on the market today drops to 200 Hz. In contrast, the AD8221 maintains a minimum common-mode rejection ratio of 80 dB to 10 kHz for all grades at G=1. The high common-mode rejection ratio overfrequency allows the AD8221 to reject broadband interference and line harmonics, greatly simplifying filter requirements. Possible applications include precision data acquisition, biomedical analysis, and aerospace instrumentation.

Low voltage offset, low offset drift, low gain drift, high gain accuracy, and high common-mode rejection ratio make this part a good choice in applications that require the best possible DC performance, such as bridge signal conditioning.

Programmable gain provides user design flexibility. A resistor sets the gain from 1 to 1000. The AD8221 operates on single and dual supplies, making it ideal for applications that encounter ±10 V input voltages.

The AD8221 features a low-cost 8-lead SOIC and 8-lead MSOP, both of which provide the best performance in the industry. MSOPs require half the board space of SOICs, making them ideal for multi-channel or space-constrained applications.

Performance for all grades is specified over the full industrial temperature range, ie -40°C to +85°C. Additionally, the AD8221 operates from -40°C to +125°C.

Typical performance characteristics

T=25°C, V=±15 V, R=10 kΩ unless otherwise noted.

theory of operation

The AD8221 is a monolithic instrumentation amplifier based on the classic three op amp topology. Input transistors Q1 and Q2 are biased at a fixed current such that any differential input signal forces the output voltages of A1 and A2 to change accordingly. A signal applied to the input generates current through R, R1 and R2 so that the outputs of A1 and A2 deliver the correct voltages. Topologically, Q1, A1, R1 and Q2, A2, R2 can be seen as precision current feedback amplifiers. The amplified differential and common-mode signals are applied to a differential amplifier that rejects the common-mode voltage but amplifies the differential voltage. The differential amplifier incorporates innovations that result in low output bias voltage and low output bias voltage drift. Laser-trimmed resistors allow high-precision amplifiers with gain errors typically less than 20ppm and common-mode rejection ratios in excess of 90db (G=1).

Using super-beta input transistors and an I compensation scheme, the AD8221 provides very high input impedance, low I, low I drift, low I, low input bias current noise, and very low voltage noise of 8 nV/√Hz. The transfer function of the AD8221 is:

The user can easily and accurately set the gain using a standard resistor.

Because the input amplifier uses a current feedback architecture, the gain-bandwidth product of the AD8221 increases with gain, resulting in a system that does not suffer the expected bandwidth penalty of a voltage feedback architecture at higher gains.

Special attention has been paid to the design and layout of the AD8221 to maintain accuracy at lower input levels, resulting in an in-amp with performance that meets the most demanding application requirements.

A unique pinout enables the AD8221 to meet the CMRR specification of 80 dB at 10 kHz (G=1) and 110 dB at 1 kHz (G=1000). The balanced pins shown in Figure 44 reduce parasitic bacteria that have adversely affected CMRR performance in the past. Additionally, the new pinout simplifies board layout because associated traces are grouped together. For example, the gain setting resistor pin is adjacent to the input and the reference pin is adjacent to the output.

Gain selection

Placing a resistor across the R terminal sets the gain of the AD8221, which can be calculated by referring to Table 6 or using the gain equation.

The AD8221 defaults to G=1 when the gain resistor is not used.

Gain accuracy is determined by the absolute tolerance of R. The TC of the external gain resistor increases the gain drift of the instrumentation amplifier. Gain error and gain drift are kept to a minimum when no gain resistors are used.

layout

Careful board layout maximizes system performance. The trace from the gain setting resistor to the R pin should be as short as possible to minimize parasitic inductance. To ensure the most accurate output, the trace of the reference pin should be connected to the local ground of the AD8221, as shown in Figure 47, or to a voltage referenced to the local ground of the AD8221.

Common Mode Rejection

One advantage of the AD8221's high common-mode rejection ratio is that it is more immune to interference, such as line noise and its associated harmonics, than typical instrumentation amplifiers. Typically, these amplifiers have CMRR attenuation at 200hz; a common mode filter is usually used to compensate for this shortcoming. The AD8221's ability to reject common-mode rejection over a wider frequency range reduces the need for filtering.

Good layout helps maintain the high common-mode rejection ratio of the AD8221 over frequency. The input source impedance and capacitance should be closely matched. Also, power supply resistors and capacitors should be placed as close to the input as possible.

ground

The output voltage of the AD8221 is derived from the potential on the reference terminal. Care should be taken to tie the REF to an appropriate local ground.

In a mixed-signal environment, low-level analog signals need to be isolated from the noisy digital environment. Many analog-to-digital converters have separate analog and digital ground pins. Although it is convenient to connect two grounds to a single ground plane, the current through the ground wire and the PC board can cause errors of hundreds of millivolts. Therefore, separate analog and digital ground loops should be used to minimize current flow from sensitive points to system ground. Figure 45 and Figure 46 show a layout example.

Reference terminal

As shown in Figure 43, the reference terminal REF is at one end of the 10 kΩ resistor. The output of the instrumentation amplifier is referenced to the voltage at the reference terminal; this is useful when the output signal needs to be offset to a precise mid-supply level. For example, a voltage source can be tied to the REF pin to level-shift the output so that the AD8221 can interface with an ADC. The allowable reference voltage range is a function of gain, input, and supply voltage. The reference pin should not exceed +V or –V by more than 0.5 V.

For best performance, the source impedance to the REF terminal should be kept low because parasitic resistance can adversely affect CMRR and gain accuracy.

Power Conditioning and Bypass

A regulated DC voltage should be used to power the instrumentation amplifier. Noise on the power pins can adversely affect performance. Bypass capacitors should be used to separate the amplifiers.

A 0.1µF capacitor should be placed near each power supply pin. As shown in Figure 47, 10µF tantalum capacitors can be used away from parts. In most cases, it can be shared by other precision integrated circuits.

Input Bias Current Return Path

The input bias current of the AD8221 must have a path back to common. When a source (such as a thermocouple) cannot provide a return path, a return path should be created, as shown in Figure 48.

input protection

All terminals of the AD8221 are ESD protected, 1kV Human Body Model In addition, the input structure allows for DC overload conditions below the negative supply -V. In negative fault conditions, an internal 400Ω resistor limits current. However, in the case of a DC overload voltage higher than the positive supply +V, a large current flows directly through the ESD diode to the positive rail. Therefore, an external resistor should be used in series with the input to limit current at voltages above +Vs. In either case, the AD8221 can safely handle 6 mA of continuous current with I=V/R for positive overvoltage and I=V/(400Ω+R) for negative overvoltage.

For applications where the AD8221 experiences extreme overload voltages, such as cardiac defibrillators, external series resistors and low leakage diode clips such as BAV199Ls, FJH1100s, or SP720s should be used.

radio frequency interference

RF correction is often an issue when amplifiers are used in applications with strong RF signals. The disturbance can appear as a small DC offset voltage. High-frequency signals can be filtered by a low-pass RC network placed at the input of the instrumentation amplifier, as shown in Figure 49. The filter limits the input signal bandwidth according to the following relationship:

where C t 10C.

C affects differential signals and C affects common mode signals. The values of R and C should be chosen to minimize RFI. A mismatch of R×C at the positive input and R×C at the negative input reduces the common-mode rejection ratio of the AD8221. By using a value that is an order of magnitude larger than C, the effect of the mismatch can be reduced, thereby improving performance.

Precision Strain Gauge

The low offset and high CMRR overfrequency characteristics of the AD8221 make it ideal for bridge measurements. As shown in Figure 50, the bridge can be connected directly to the input of the amplifier.

Conditioning ±10 V Signals for A+5 V Differential Input ADCs

Mode rejection, noise immunity, and performance at low supply voltages. Connecting a ±10 V single-ended instrumentation amplifier to a +5 V differential ADC can be a challenge. Connecting an instrumentation amplifier to an ADC requires attenuation and level shifting. The solution is shown in Figure 51.

±10 V signals need to be conditioned in many applications. However, many ADCs and digital ICs today operate at lower single supply voltages. Also, the new ADCs have differential inputs as they provide better common - in this topology the OP27 sets the reference voltage for the AD8221. The output signal of the instrumentation amplifier is received through the OUT pin and the REF pin. Two 1 kΩ resistors and one 499Ω resistor attenuate ±10 V signals to +4 V. Optional capacitor C1 can be used as an anti-aliasing filter. AD8022 is used to drive the ADC.

This topology has five advantages. Apart from level shift and attenuation, the contribution to the system is very small. Noise from R1 and R2 is common to both ADC inputs and is easily rejected. R5 adds a third of the dominant noise, so the contribution to the system noise is negligible. The attenuator separates the noise from R3 and R4. Also, its noise contribution is negligible. A fourth advantage of this interface circuit is that the capture time of the AD8221 is reduced by a factor of 2. With the help of the OP27, the AD8221 only needs to provide half of its full load; therefore, the signal settles faster. Finally, it helps that the AD8022 does this very quickly, as the shorter the processing time, the more bits can be parsed while the ADC acquires the data. This configuration provides attenuation, level shifting, and easy interfacing with differential input ADCs while maintaining performance.

AC Coupled Instrumentation Amplifier

Measuring small signals in amplifier noise or offset can be a challenge. Figure 52 shows a circuit that can improve the resolution of small AC signals. The large gain reduces the amplifier's input noise to 8nv/√Hz. Therefore, smaller signals can be measured because the noise floor is lower. The integrator feedback network removes the dc offset that should be 100 from the output of the AD8221.

At low frequencies, the OP1177 forces the output of the AD8221 to 0V. Once the signal exceeds f, the AD8221 outputs the amplified input signal.

Mold information

Die size: 1575 microns x 2230 microns

Die thickness: 381 microns

To reduce the gain error introduced by the bond wires, a Kelvin connection is used between the chip and the gain resistor R by placing pads 2A and 2B parallel to one end of R and pads 3A and 3B parallel to the other end of the R. For unity gain applications where R is not required, pads 2A and 2B and pads 3A and 3B must be bonded together.

Dimensions