ZHCSJ65A December   2018  – December 2019 OPA462

PRODUCTION DATA.  

  1. 特性
  2. 应用
  3. 说明
    1.     Device Images
      1.      OPA462 方框图
      2.      最大输出电压与频率间的关系
  4. 修订历史记录
  5. Pin Configuration and Functions
    1.     Pin Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics: Table of Graphs
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Status Flag Pin
      2. 7.3.2 Thermal Protection
      3. 7.3.3 Current Limit
      4. 7.3.4 Enable and Disable
    4. 7.4 Device Functional Modes
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 High DAC Gain Stage for Semiconductor Test Equipment
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
      2. 8.2.2 Improved Howland Current Pump for Bioimpedance Measurements in Multiparameter Patient Monitors
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Thermally-Enhanced PowerPAD Package
      2. 10.1.2 PowerPAD Integrated Circuit Package Layout Guidelines
      3. 10.1.3 Pin Leakage
      4. 10.1.4 Thermal Protection
      5. 10.1.5 Power Dissipation
      6. 10.1.6 Heat Dissipation
    2. 10.2 Layout Example
  11. 11器件和文档支持
    1. 11.1 器件支持
      1. 11.1.1 开发支持
        1. 11.1.1.1 TINA-TI™(免费软件下载)
        2. 11.1.1.2 TI 高精度设计
        3. 11.1.1.3 WEBENCH滤波器设计器
    2. 11.2 文档支持
      1. 11.2.1 相关文档
    3. 11.3 支持资源
    4. 11.4 商标
    5. 11.5 静电放电警告
    6. 11.6 Glossary
  12. 12机械、封装和可订购信息

封装选项

机械数据 (封装 | 引脚)
散热焊盘机械数据 (封装 | 引脚)
订购信息

Detailed Design Procedure

The improved Howland current pump circuit is illustrated in Figure 63. The OPA462 sources an output current of 20 mA when a low-voltage single-ended, 2.5-V reference voltage is applied to the circuit input. The source could be an actual 2.5-V precision reference. If the current-pump output current requires being set to different levels, a voltage output DAC can be used. If the input voltage polarity is reversed, the output current reverses direction, and 20 mA is sunk from the load through the OPA462 output.

The circuit shown in Figure 63 provides the resistance values required to obtain a ±20-mA output current with the 2.5-V input voltage applied. The following can be used to select the resistors, thus setting the voltage gain and output current.

  • R13 sets the gain, and is adjusted by the ratio of R14 / R15
  • Selecting a low value for R13 enables all other resistors to be high, limiting current through the feedback network
  • The ratio of R11 / (R12 + R13) must equal R14 / R15
  • If R14 = R15, then R12 = R11 – R13

Applying these relationships the resistors are selected or derived as follows:

  • Let R14 = R15 = 100 kΩ
  • R13 = [(VP – VN) (R15 / R14)] / IL = [(2.5 V – 0 V) (105 Ω / 105 Ω)] = 125 Ω
  • R12 = (R11 – R13) = (100 kΩ – 125 Ω) = 99.875 kΩ

Verifying R11 / (R12 + R13) must equal R14 / R15 requirement:

  • R12 = [R11(R15 / R14)] – R13 = [105 Ω (105 Ω / 105 Ω)] – 125 Ω = 99.875 kΩ

The resistor values for R11 through R15 are seen in Figure 63.

The load is set to be 500 Ω, the sourced output current through the load is 20 mA, and the output voltage is 10 V. The voltage directly at the OPA462 output 2.5 V higher, or 12.5 V, which compensates for the voltage drop across the 125-Ω R13 resistor. A feedback capacitor can be added to reduce the ac bandwidth of the improved Howland current pump circuit, if needed. In this example, no capacitor is used.

The improved Howland current pump output is limited to the combined effects of the OPA462 linear output voltage swing range, the voltage drop developed across R13, and the voltage drop developed across load. For a particular output current, a maximum output voltage span can be achieved. This span is referred to as the output voltage compliance range.

The OPA462 current pump sources or sinks a constant current through a load resistance of 0 Ω on the low end, to just beyond 4.25 kΩ on the high end. This current range is portrayed in the dc transfer plot show in Figure 64. As shown, the load can be vary from 0 Ω to 4.25 kΩ and the output remains within the span of linear output compliance range.

OPA462 opa462-output-voltage-compliance-for-an-improved-howland-current.pngFigure 64. Output Voltage Compliance for an Improved Howland Current Pump

The 4.25-kΩ limit is determined by the maximum 85-V drop across the load, and the 2.5-V drop across R13 when 20 mA flows through both. This voltage drop results in an output voltage of 87.5 V at the output of the OPA462, close to the positive swing limit. Beyond 4.25 kΩ, current-pump operation is forced outside the compliance range, and the output current is longer maintained at the correct level.

The OPA462 provides this wide output compliance range because of the wide, ±90-V power supply rating. If a standard ±15-V amplifier supply had been used with the OPA462, or another amplifier rated for ±15-V supplies, the maximum load resistance is on the order of approximately 500 Ω to 600 Ω, depending on the particular amplifier linear output range when delivering ±20 mA. The wide supply range of the OPA462 enables the device to drive a much wider range of loads.

The improved Howland current pump can also be used to generate an accurate ac current with a peak output that matches a specified dc current level. A ±20-mA dc current source using the OPA462 has already been discussed; therefore, this current source is applied here to demonstrate how a 400-Hz, 20-mA current is produced.

The same circuit used in Figure 63 is updated so that the 2.5-V dc voltage source has been replaced by a 400-Hz ac source with a peak voltage of 2.5 V, as shown in Figure 65. A sine wave is used in this circuit, but a triangle wave, square wave, and so on, can be used instead. The output current is dependent on the ac input voltage at any particular moment.

OPA462 opa462-configured-as-a-400-hz-ac-current-generator.gifFigure 65. OPA462 Configured as a 400-Hz AC Current Generator

A 2.5-Vpk sine-wave source applied to the input point at R11 results in a 20-mA peak current through the load, as shown in Figure 66. The load has been set to 1 kΩ, but any resistance that supports the output compliance range can be used.

OPA462 opa462-improved-howland-current-pump-applied-as-a-peak-ac-current-generator.pngFigure 66. Improved Howland Current Pump Applied as a Peak AC-Current Generator

Make sure to consider the power handling ratings of the resistors used with a high-power or high-voltage amplifier such as the OPA462. In this design, when the OPA462 is providing 20 mA dc to a 4.25-kΩ load resistance, the dc power for the load and R13 is simply:

  • Load Power = I2 ∙ RL = (20 ∙ 10 – 3 A)2 (4.25 ∙ 103 Ω) = 1.7 W
  • Power R13 = I2 ∙ R13 = (20 ∙ 10 – 3 A)2 (125 Ω) = 50 mW

Clearly, the power dissipation of the load requires attention. However, in this design, R13 does not require high power dissipation under these operating conditions. The load must be rated to dissipate the 1.7 W over the expected operating temperature range for this example. Most often, resistor power dissipation is specified at an ambient temperature of 25°C, and reduces as temperature increases. The use of a resistor with a power rating greater than the power that must be dissipated is almost always necessary. For this example, the load may need to be rated for 3 W, or even 5 W, to make sure that the load does not overheat and maintains reliability. in any case, determine the power dissipation for the particular operating conditions. Be especially attentive to the power rating issue regarding surface-mount resistors. The thermal environment in which surface-mount resistors operate may be much different than a resistor exposed in free air.

The improved Howland current pump amplifier circuit relies on both negative and positive feedback for operation. More negative feedback than positive feedback is used, but that does not always provide stability when the output load characteristics are included. When unity-gain stable amplifiers such as the OPA462 are employed, and they drive a resistive load, the amplifier phase margin should be sufficient so that the circuit is stable. However, if the output load is complex, containing both resistive and reactive components (R±jX), certain combinations degrade the phase margin to the point where instability results. Instability is even more evident when this current pump is used to drive certain inductive loads.

When required, compensation is determined based on the particular circuit to which the OPA462 is being applied. Amplifier stability and compensation is a vast subject covered in numerous TI documents, and TI training programs, such as TI Precision Labs – Op Amps.