8.2.2.1 Setting the Gain

A single external resistor (R_G) that is connected between pins 1 and 8 sets the gain of the INA317. The value of R_G is selected according to Equation 1:

Table 1 lists several commonly-used gains and resistor values. The 100 kΩ in Equation 1 is a result of the sum of the two internal feedback resistors (A₁ and A₂.) These on-chip resistors are laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these resistors are included in the gain accuracy and drift specifications of the INA317 device.

The stability and temperature drift of the external gain setting resistor (R_G) also affects gain. The contribution of R_G to gain accuracy and drift is inferred from the gain inEquation 1. Low resistor values required for high gain make wiring resistance important. Sockets add to the wiring resistance and contribute additional gain error (possibly an unstable gain error) in gains of approximately 100 or greater. To ensure stability, avoid parasitic capacitance of more than a few picofarads at the R_G connections. Careful matching of any parasitics on R_G pins maintains optimal CMRR over frequency.

8.2.2.2 Internal Offset Correction

The INA317 device internal operational amplifiers use an autocalibration technique with a time-continuous 350-kHz operational amplifier in the signal path. The amplifier is zero-corrected every 8 µs using a proprietary technique. At power-up, the amplifier requires approximately 100 µs to achieve specified V_OS accuracy. This design has no aliasing or flicker noise.

8.2.2.3 Offset Trimming

Most applications require no external offset adjustment. However, apply a voltage to the REF pin to make adjustments if necessary. Figure 33 shows an optional circuit for trimming the output offset voltage. The voltage applied to REF pin is added at the output. The operational amplifier buffer provides low impedance at the REF pin to preserve good common-mode rejection.

Figure 33. Optional Trimming of Output Offset Voltage

8.2.2.4 Noise Performance

The autocalibration technique used by the INA317 device results in reduced low-frequency noise, typically only 50 nV/√Hz (G = 100). The spectral noise density is shown in Figure 8. Low-frequency noise of the INA317 device is approximately 1 µV_PP measured from 0.1 Hz to 10 Hz (G = 100).

8.2.2.5 Input Bias Current Return Path

The input impedance of the INA317 device is extremely high(approximately 100 GΩ.) However, a path must be provided for the input bias current of the inputs. This input bias current is typically ±70 pA. High-input impedance means that this input bias current changes very little with varying input voltage.

For proper operation, input circuitry must provide a path for the input bias current. Figure 34 shows various provisions for an input bias current path. Without a bias current path, the inputs float to a potential that exceeds the common-mode range of the INA317 device, and the input amplifiers saturate. If the differential source resistance is low, the bias current return path connects to one input (see the thermocouple example in Figure 34). With higher source impedance, using two equal resistors provides a balanced input with possible advantages of lower input offset voltage as a result of bias current and better high-frequency common-mode rejection.

Figure 34. Providing an Input Common-Mode Current Path

8.2.2.6 Input Common-Mode Range

The linear input voltage range of the input circuitry of the INA317 device is from approximately 0.1 V below the positive supply voltage to 0.1 V above the negative supply. As a differential input voltage causes the output voltage to increase, however, the linear input range is limited by the output voltage swing of amplifiers A₁ and A₂. The linear common-mode input range is related to the output voltage of the complete amplifier. This behavior depends on supply voltage(see Figure 20 to Figure 23 in the Typical Characteristics section.)

Input overload conditions can produce an output voltage that appears normal. For example, if an input overload condition drives both input amplifiers to the respective positive output swing limit, the difference voltage measured by the output amplifier is near zero. The output of the INA317 is near 0 V even though both inputs are overloaded.

8.2.2.7 Operating Voltage

The INA317 operates over a power-supply range of 1.8 V to 5.5 V (±0.9 V to ±2.75 V). Supply voltages higher than 7 V (absolute maximum) can permanently damage the device. Parameters that vary over supply voltage or temperature are shown in the Typical Characteristics section of this data sheet.

8.2.2.8 Low Voltage Operation

The INA317 device operates on power supplies as low as ±0.9 V. Most parameters vary only slightly throughout this supply voltage range; see the Typical Characteristics section. Operation at very low supply voltage requires careful attention to ensure that the input voltages remain within the linear range. Voltage swing requirements of internal nodes limit the input common-mode range with low power supply voltage. Figure 20 to Figure 23 show the range of linear operation for various supply voltages and gains.

8.2.2.9 Single-Supply Operation

The INA317 device can be used on single power supplies of 1.8 V to 5.5 V. Figure 35 shows a basic single-supply circuit. The output REF pin is connected to midsupply. Zero differential input voltage demands an output voltage of midsupply. Actual output voltage swing is limited to approximately 50 mV more than ground when the load is referred to ground as shown. Figure 29 shows how the output voltage swing varies with output current.

With single-supply operation, V_IN+ and V_IN– must be 0.1 V more than ground for linear operation. For instance, the inverting input cannot connect to ground to measure a voltage that is connected to the noninverting input.

To show the issues affecting low voltage operation, see Figure 35. Figure 35 shows the INA317 device operating from a single 3-V supply. A resistor in series with the low side of the bridge ensures that the bridge output voltage is within the common-mode range of the amplifier inputs.

1. R₁ creates proper common-mode voltage only for low-voltage operation; see Single-Supply Operation.

Figure 35. Single-Supply Bridge Amplifier

8.2.2.10 Input Protection

The input pins of the INA317 device are protected with internal diodes that are connected to the power-supply rails. These diodes clamp the applied signal to prevent the signal from damaging the input circuitry. If the input signal voltage exceeds the power supplies by more than 0.3 V, the input signal current must be limited to less than 10 mA to protect the internal clamp diodes. Limit the current with a series input resistor. Some signal sources are inherently current limited and do not require limiting resistors.