feature

Common Mode Input Range: 00V (VS=?5V) Protection Input: 00V Common Mode ±500V Differential Unit Gain: Maximum Gain Error 0.02% Nonlinearity: Maximum 0.001% Common Mode Rejection Ratio: Minimum 86dB

application

Current monitor

battery voltage monitor

ground circuit breaker

input protection

Signal acquisition environment in noise

factory automation

illustrate

The INA117 is a precision unity gain differential high range common mode input voltage amplifier. It is a monolithic integrated circuit consisting of a precision operational amplifier with an integrated thin film resistor network. It can accurately measure the rising voltage of small differential common mode signal to ± 200V . The INA117 inputs are protected against transient common-mode or differential overloads up to ±500V. In many applications where galvanic isolation is not essential, the INA117 can replace an isolation amplifier. This eliminates expensive isolated input-side power supplies and their associated ripple, noise, and static current. The INA117's 0.001% nonlinearity and 200kHz bandwidth outperform conventional isolation amplifiers. The INA117 is available in 8-pin miniature plastic dipping and SO-8 surface mount components for the -40°C to +85°C temperature range. Metal-99 models are suitable for temperature ranges from -40°C to +85°C and –55°C to + 125 °C.

VS=±15V at TA=+25°C unless otherwise noted

Specifications are the same as INA117AM.

Note: (1) Connect as a differential amplifier (see Figure 1). (2) Nonlinearity is the maximum peak-to-peak deviation from the best-fit straight line, as a percentage of full scale peak-to-peak output. (3) Zero source impedance (see Common Mode Rejection discussion in the Applications Information section). (4) Including the effects of amplifier input bias and bias current. (5) Includes the effects of amplifier input current noise and resistor network thermal noise contributions.

application information

Figure 1 shows the basic connections required for operation. Applications with noisy or high impedance power lines may require decoupling capacitors close to the device pins. The output voltage is equal to the differential input voltage between pins 2 and 3. Common mode input voltage is rejected. An internal circuit connected to the compensation pin 8 can control the parasitic distributed capacitance between the feedback resistor R2 and the IC substrate. For specified dynamic performance, pin 8 should be grounded or connected to AC ground through a 0.1µF capacitor, such as V+

Common Mode Rejection

The common mode rejection (CMR) of the INA117 depends on the input resistor network, which is laser trimmed for precise ratio matching. To maintain a high CMR, it is important to drive both inputs with low source impedance. A 75Ω resistor in series with pin 2 or 3 will reduce the CMR from 86dB to 72dB. A resistor in series with the reference pin will also degrade CMR. A 4Ω resistor in series with pin 1 or 5 will reduce the CMRR from 86dB to 72dB. Most applications do not require pruning. Figures 2 and 3 show optional circuit voltages and common-mode rejection that can be used to trim the offset. Transfer Function Most applications use the INA117 as a simple unity gain differential amplifier. The transfer function is:

However, some applications apply voltage terminals (pins 1 and 5) to the reference voltage. A more complete transfer function is:

measure current

The INA117 can sense the voltage drop across the series resistor, as shown in Figure 4. The INA117 is being tested to measure the device supply current. The circuit in Figure 5 measures the current of the output power supply. If the power supply has an inductive connection, it can be connected to the output side of RS to eliminate voltage drop errors. Another common application is current-to-voltage conversion, as shown in Figure 6.

In any case, the sense resistor will unbalance the input resistor matching the INA117, reducing its CMR. In addition, the impedance of the input INA117 load RS, resulting in voltage-current conversion. Both errors are probably easy to correct. This can be done by adding a compensation resistor, RC, equal to RS, as shown in Figures 4, 5 and 6. If RS is less than 20Ω, the degradation is negligible in CMR and RC can be omitted. If RS is larger than about 2kΩ, RC may need to be trimmed to achieve greater than 86dB CMR. This is because the INA117 input impedance has a typical mismatch of 1%. If RS is greater than about 100Ω, the gain error will be greater than the INA117's 0.02% specification. This gain error can be corrected for RS by slightly increasing this value. The correction value RS' can be calculated by the following formula:

Example: For a 1V/mA transfer function, nominally, the uncorrected value of RS is 1kΩ. A slightly larger value RS'=1002.6Ω compensates due to loading. The 380kΩ term in the RS' equation has a tolerance of ±25%, so sense resistors above about 400Ω may need trimming to achieve better than 0.02% accuracy. Of course, if you add buffer amplifier 7 as shown, both inputs see low source impedance and the resistors are not loaded. Therefore, there is no gain error or CMR degradation. The buffer amplifier can act as a unity gain buffer or an amplifier with non-inverting gain. The added gain before the INA117 improves CMR and signal-to-noise ratio. The increased gain also reduces the voltage drop across the sense resistor. The OPA1013 is a good choice for a buffer amplifier because both its input and output can swing close to its negative supply.

Figure 8 shows the

Measures low leakage current. Here, the buffer op amp is powered by an isolated voltage divider supply. Using isolated power supplies allows for a full ±200V common-mode input range. Noise Performance The noise performance of the INA117 is mainly determined by the internal resistor network. Thermal Noise or Johnson Noise These resistors generate noise around 550nV/√Hz. Internal op-amp at frequencies above 100Hz. Many applications may be satisfied with the INA117's 200kHz bandwidth. In these cases, the noise can be reduced with a low-pass filter at the output. The two-pole filter shown in Figure 9 limits the bandwidth to 1kHz and reduces noise by more than 15:1. Because the INA117 has a 1/f noise corner frequency of about 100Hz, a cutoff of frequencies below 100Hz will not further reduce noise.