SGLS185F September 2003 – May 2016 INA139-Q1 , INA169-Q1
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
-
8 Application and Implementation
- 8.1 Application Information
- 8.2 Typical Applications
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information
8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Operation
Figure 9 illustrates the basic circuit diagram for both the INA139-Q1 and INA169-Q1. Load current IS is drawn from supply VP through shunt resistor RS. The voltage drop in shunt resistor RS is forced across RG1 by the internal op amp, causing current to flow into the collector of Q1. External resistor RL converts the output current, IO, to a voltage, VOUT, at the OUT pin. The transfer function for the INA1x9-Q1 is given by Equation 3:
where
- where gm = 1000 μA/V.
In the circuit of Figure 9, the input voltage (VIN+ − VIN−) is equal to IS × RS and the output voltage (VOUT) is equal to IO × RL. The transconductance (gm) of the INA1x9-Q1 is 1000 μA/V. The complete transfer function for the current measurement amplifier in this application is given by Equation 4:
The maximum differential input voltage for accurate measurements is 0.5 V, producing a 500-μA output current. A differential input voltage of up to 2 V does not cause damage. Differential measurements (VIN+ and VIN− pins) must be unipolar, with a more-positive voltage applied to the VIN+ pin. If a more-negative voltage is applied to VIN+ pin, IO goes to zero, but no damage occurs.
Table 1. Voltage Gains and Corresponding Load-Resistor Values
| VOLTAGE GAIN | EXACT RL (kΩ) | NEAREST 1% RL (kΩ) |
|---|---|---|
| 1 | 1 | 1 |
| 2 | 2 | 2 |
| 5 | 5 | 4.99 |
| 10 | 10 | 10 |
| 20 | 20 | 20 |
| 50 | 50 | 49 |
| 100 | 100 | 100 |
8.2 Typical Applications
The INA1x9-Q1 are designed for current-shunt measurement circuits, as shown in Figure 9, but the basic function is useful in a wide range of circuitry. With a little creativity, many unforeseen uses are found in measurement and level shifting circuits. A few ideas are illustrated in the following subsections.
8.2.1 Buffering Output to Drive an ADC
Digitize the output of the INA139-Q1 or INA169-Q1 devices using a 1-MSPS analog-to-digital converter (ADC).
8.2.1.1 Design Requirements
For this design example, use the input parameters shown in Table 2.
Table 2. Design Parameters
| DESIGN PARAMETER | EXAMPLE VALUE |
|---|---|
| Supply voltage, V+ | 5 V |
| Common-mode voltage, VCM | INA139-Q1: 2.7 V to 40 V |
| INA169-Q1: 2.7 V to 60 V | |
| Full-scale shunt voltage, VSENSE | 50 mV to 100 mV |
| Load resistor, RL | 1 kΩ to 100 kΩ |
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Selecting RS and RL
In Figure 10, the value chosen for the shunt resistor, RS, depends on the application and is a compromise between small-signal accuracy and maximum permissible voltage loss in the measurement line. High values of RS provide better accuracy at lower currents by minimizing the effects of offset, while low values of RS minimize voltage loss in the supply line. For most applications, best performance is attained with an RS value that provides a full-scale shunt voltage range of 50 mV to 100 mV. Maximum input voltage for accurate measurements is 500 mV.
Choose an RL that provides the desired full-scale output voltage. The output impedance of the INA1x9-Q1 OUT pin is very high, permitting the use of RL values up to 100 kΩ with excellent accuracy. The input impedance of any additional circuitry at the output must be much higher than the value of RL to avoid degrading accuracy.
Some ADCs have input impedances that significantly affect measurement gain. The input impedance of the A/D converter can be included as part of the effective RL if its input can be modeled as a resistor to ground. Alternatively, an op amp can be used to buffer the ADC input, as shown in Figure 10. The INA1x9-Q1 are current output devices, and as such have an inherently large output impedance. The output currents from the amplifier are converted to an output voltage using the load resistor, RL, connected from the amplifier output to ground. The ratio of the load resistor value to that of the internal resistor value determines the voltage gain of the system.
In many applications, digitizing the output of the INA1x9-Q1 is required. Digitizing is accomplished by connecting the output of the amplifier to an ADC. It is very common for an ADC to have a dynamic input impedance. If the INA1x9-Q1 output is connected directly to an ADC input, the input impedance of the ADC is effectively connected in parallel with gain setting resistor RL. This parallel impedance combination affects the gain of the system and the impact on the gain is difficult to estimate accurately. A simple solution that eliminates the paralleling of impedances, and simplifies the gain of the circuit is to place a buffer amplifier, such as the OPA340, between the output of the INA1x9-Q1 and the input to the ADC.
Figure 10 illustrates this concept. Notice that a low-pass filter is placed between the OPA340 output and the input to the ADC. The filter capacitor is required to provide any instantaneous demand for current required by the input stage of the ADC. The filter resistor is required to isolate the OPA340 output from the filter capacitor in order to maintain circuit stability. The values for the filter components vary according to the operational amplifier used for the buffer and the particular ADC selected. More information regarding the design of the low-pass filter is found in the TI Precision Design, 16 bit 1MSPS Data Acquisition Reference Design for Single-Ended Multiplexed Applications, TIPD173.
Figure 11 shows the expected results when driving an ADC at 1 MSPS with and without buffering the INA1x9-Q1 output. Without the buffer, the high impedance of the INA1x9-Q1 reacts with the input capacitance and sample-and-hold capacitance of the ADC, and does not allow the sampled value to reach the correct final value before the ADC is reset, and the next conversion starts. Adding the buffer amplifier significantly reduces the output impedance driving the sample-and-hold circuitry, and allows for higher conversion rates.
8.2.1.3 Application Curve
Figure 11. Driving an ADC With and Without a Buffer
8.2.2 Output Filter
Filter the output of the INA139-Q1 or INA169-Q1 devices.
Figure 12. Output Filter
8.2.2.1 Design Requirements
For this design example, use the input parameters shown in Table 3.
Table 3. Design Parameters
| DESIGN PARAMETER | EXAMPLE VALUE |
|---|---|
| Supply voltage, V+ | INA139-Q1: 0 V to 40 V |
| INA169-Q1: 0 V to 60 V | |
| Common-mode voltage, VCM | INA139-Q1: 0 V to 40 V |
| INA169-Q1: 0 V to 60 V | |
| Full-scale shunt voltage, VSENSE | 50 mV to 100 mV |
| Load resistor, RL | 1 kΩ to 100 kΩ |
8.2.2.2 Detailed Design Procedure
A low-pass filter can be formed at the output of the INA1x9-Q1 simply by placing a capacitor of the desired value in parallel with the load resistor. First, determine the value of the load resistor needed to achieve the desired gain by using Table 1. Next, determine the capacitor value that results in the desired cutoff frequency according to the equation shown in Figure 12. Figure 13 shows the frequency response with different RL values and a fixed filter capacitor.
8.2.2.3 Application Curve
8.2.3 Offsetting the Output Voltage
For many applications using only a single power supply, the output voltage may have to be level shifted away from ground when there is no load current flowing in the shunt resistor. Level shifting the output of the INA1x9-Q1 is easily accomplished by one of two simple methods shown in Figure 14. Method (a) on the left-hand side of Figure 14 illustrates a simple voltage divider method. This method is useful for applications that require the output of the INA1x9-Q1 to remain centered with respect to the power supply at a zero load current through the shunt resistor. Using this method, the gain is determined by the parallel combination of R1 and R2, while the output offset is determined by the voltage divider ratio of R1 and R2, as shown in Figure 14(a). For applications that require a fixed value of output offset independent of the power supply voltage, use current-source method (b), shown on the right-hand side of Figure 14. With this method, a REF200 constant current source is used to generate a constant output offset. Using his method, the gain is determined by RL, and the offset is determined by the product of the value of the current source and RL.
8.2.4 Bipolar Current Measurement
Configure the INA1x9-Q1 as shown in Figure 15 for applications where bidirectional current measurement is required. Two INA1x9-Q1 devices are required; connect the inputs across the shunt resistor as shown in Figure 15. A comparator, such as the TLV3201, is used to detect the polarity of the load current. The magnitude of the load current is monitored across the resistor connected between ground and the connection labeled Output. In this example, the 20-kΩ resistor results in a gain of 20 V/V. The 10-kΩ resistors connected in series with the INA1x9-Q1 output current are used to develop a voltage across the comparator inputs. Two diodes are required to prevent current flow into the INA1x9-Q1 output because only one device at a time provides current to the Output connection of the circuit. The circuit functionality is illustrated in Figure 16.
8.2.4.1 Application Curve
Figure 16. Bipolar Current Measurement Results (Arbitrary Scale)
8.2.5 Bipolar Current Measurement Using Differential Input of the ADC
Use the INA1x9-Q1 with an ADC, such as the ADS7870, programmed for differential-mode operation; Figure 17 illustrates this configuration. In this configuration, the use of two INA138-Q1s or INA168-Q1s allows for bidirectional current measurement. Depending on the polarity of the current, one of the INA devices provides an output voltage, while the other INA device output is zero. In this way, the ADC reads the polarity of current directly, without the need for additional circuitry.
Figure 17. Bipolar Current Measurement Using Differential Input of the ADC
8.2.6 Multiplexed Measurement Using Logic Signal for Power
Measure multiple loads as shown in Figure 18. In this configuration, each INA1x9-Q1 device is powered by the digital I/O from the ADS7870. Multiplexing is achieved by switching on or off each desired I/O.
Figure 18. Multiplexed Measurement Using Logic Signal for Power