Precision Instrument Amplifier INA114

Precision Instrument Amplifier INA114

INA114 is a general-purpose instrument amplifier with small size, high precision and low price. It can be used for bridges, thermocouples, data acquisition, RTD sensors and medical instruments. INA114 only needs one external resistor to set any gain value between 1 and 10000. The internal input protection can withstand ±40V for a long time, with low offset voltage (50μV), small drift (0.25μV/℃), high common mode rejection ratio (50dB when G=1000), laser adjustment, and can work at a voltage of ±2.25V. It uses a battery (group) or a 5V single power supply system, and the maximum quiescent current is 3mA. INA114 uses an 8-pin plastic package or SOL-16 surface mount package, and the operating environment temperature is -40℃～+85℃.

The internal circuit and pin layout of INA114 are shown in Figures 1 and 2.

Figure 1: Internal circuit diagram of INA114

Figure 2: INA114 pinout

2. Main electrical parameters

Table 1 shows the main electrical parameters of INA114.

Since INA114 has many parameters, only some of them are listed here. More detailed information can be downloaded from BB company's homepage.

The absolute maximum usage limits of the INA114 are given below.

Supply voltage: ±18V

Input voltage range: ±40V

Working temperature: -40℃～+125℃

Storage temperature: -40℃～+125℃

Node temperature: +150℃

Pin temperature (soft soldering, 10s): +300℃

3. Application Design and Examples

Figure 3: Basic interface

Figure 3 shows the basic connection requirements for the INA114. If used with a noisy or high-impedance power supply, decoupling capacitors need to be connected close to the power supply pins.

Build Gain

The gain value of INA114 can be set with only one external resistor RG, as shown below:

G=1+50kΩ/RG (1)

Figure 3 shows commonly used gain values ​​and corresponding resistance values, and Table 2 lists commonly used gain values ​​and approximate RG resistance values ​​for INA114.

The term "50 kΩ" in equation (1) is the sum of two internal feedback resistors. These two metal film resistors have been laser trimmed to precise values. The accuracy and temperature coefficients of these two resistors are included in the accuracy and drift ratings of the INA114 gain.

The stability and temperature drift of the external resistor RG used to set the gain also affect the gain. The effect of RG on gain accuracy and gain drift can be directly derived from formula (1). High gain requires low resistance, so wiring resistance is important. Adding sockets to the line will increase the gain error by an additional 100 or more, and it is likely to be an unstable error.

Noise characteristics

The INA114 generates very little noise in most applications. The INA103 generates even less noise for differential source resistances less than 1 kΩ, and the INA111 FET-input instrumentation amplifier generates even less noise for source resistances greater than 5 kΩ.

The INA114's low-frequency noise is about 0.4 μV peak-to-peak from 0.1 Hz to 10 Hz. This is about one-tenth the noise produced by a "low-noise" amplifier that uses chopper stabilization.

Offset/Deviation Correction

INA114 uses laser to correct small offset voltage and drift, and no external offset correction is required in most applications. Figure 4 shows a loop to correct the output deviation voltage. Low resistance must be used at this node to ensure good common-mode rejection ratio, which can be achieved by buffering with an operational amplifier.

Figure 4: Output deviation correction circuit

Bias Voltage Return Path

The input resistance of INA114 is very large (about 1010Ω), but a return path must be set for the bias current of both input terminals. The bias current is less than ±1nA (due to the influence of the circuit, the bias current can be positive or negative). Because the input resistance is large, the change of bias current is very small when the input voltage changes.

Figure 5: Providing a bias supply return path

An input current return path must be provided for the INA114 to operate properly. Figure 5 shows some designs that provide a bias current return path. Without a bias current return path, the input will float beyond the common-mode range of the INA114 and the amplifier will saturate. If the differential source resistance is small, the bias current path can be connected to one pin (see the thermocouple in Figure 5). When the source resistance is large, two equal resistors provide a balanced input, and the bias voltage will be lower due to the bias current and good common-mode rejection ratio.

Input common mode range

The linear common-mode range of the INA114 op amp is related to the output voltage of the entire amplifier, which is approximately ±13.75V (or 1.25V from the power supply voltage). As the output voltage increases, the output voltage swing of the input op amps A1 and A2 limits the linear output range.

Figure 6: Voltage swings of A1 and A2

The combination of common-mode and differential input signals can cause the output of A1 or A2 to saturate. Figure 6 shows the output voltage swing of A1 and A2 as a function of common-mode and differential input voltages. These internal amplifiers have the same output swing capability as the external amplifier A3. In applications where the input common-mode needs to have the maximum range, set the INA114 to a smaller gain to limit the output voltage swing. If necessary, add more gain after the INA114 to increase the output voltage swing.

The output often behaves normally when the input is overloaded. For example, when one input is at +20V and the other at +40V, both inputs are clearly beyond the linear common-mode range of the amplifier. Since both amplifiers are saturated and close to the same output voltage limit, the differential voltage measured by the output voltage is close to zero, so if both inputs are overloaded, the output of the INA114 is close to zero.

Input protection

The protection of the two inputs of INA114 can withstand ±40V voltage each. Even if the voltage of one input is -40V and the other is +40V, there will be no danger. The internal circuit of each input provides a small series resistance under normal signal conditions. To provide equivalent protection, the series input resistance will generate excessive noise. If the input is overloaded, the protection circuit limits the input current to a safe value (about 1.5mA). The input is still protected even if there is no power supply voltage.

Figure 7: Long-distance load and ground sensing

Output voltage detection (only for SOL-16 package)

The surface mount version of INA114 has an independent output detection feedback interface, pin 12, which must be connected to pin 11 for operation. In the DIP version of INA114, this interface is located inside the chip.

The output sense interface can be used to sense the output voltage directly under load conditions for best accuracy. Figure 7 shows how the load is driven through an internally connected resistor. Remotely located feedback paths can cause instability, which can be eliminated by using a high frequency feedback path through C1.

Figures 7 and 8 are typical circuits of two applications for readers' reference.

Conclusion

In summary, the INA114 precision instrument amplifier has high accuracy, large gain range, excellent performance and low price, making it very suitable for use in precision instruments.