SBOS513F August   2010  – December 2016 OPA2320 , OPA320

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and 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: OPA320 and OPA320S
    5. 6.5 Thermal Information: OPA2320
    6. 6.6 Thermal Information: OPA2320S
    7. 6.7 Electrical Characteristics
    8. 6.8 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Operating Voltage
      2. 7.3.2  Input and ESD Protection
      3. 7.3.3  Rail-to-Rail Input
      4. 7.3.4  Phase Reversal
      5. 7.3.5  Feedback Capacitor Improves Response
      6. 7.3.6  EMI Susceptibility and Input Filtering
      7. 7.3.7  Output Impedance
      8. 7.3.8  Capacitive Load and Stability
      9. 7.3.9  Overload Recovery Time
      10. 7.3.10 Shutdown Function
      11. 7.3.11 Leadless SON Package
    4. 7.4 Device Functional Modes
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Transimpedance Amplifier
      2. 8.1.2 Optimizing the Transimpedance Circuit
      3. 8.1.3 High-Impedance Sensor Interface
      4. 8.1.4 Driving ADC'S
      5. 8.1.5 Active Filter
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
      3. 8.2.3 Application Curve
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 TINA-TI™ (Free Software Download)
        2. 11.1.1.2 DIP Adapter EVM
        3. 11.1.1.3 Universal Op Amp EVM
        4. 11.1.1.4 TI Precision Designs
        5. 11.1.1.5 WEBENCH® Filter Designer
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Related Links
    4. 11.4 Receiving Notification of Documentation Updates
    5. 11.5 Community Resource
    6. 11.6 Trademarks
    7. 11.7 Electrostatic Discharge Caution
    8. 11.8 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

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.

Application Information

The OPA320 family offers outstanding DC and AC performance. These devices operate up to a 5.5-V power supply and offer ultra-low input bias current and 20-MHz bandwidth. These features make the OPA320 family a robust operational amplifier for both battery-powered and industrial applications.

Transimpedance Amplifier

Wide gain bandwidth, low-input bias current, low input voltage, and current noise make the OPA320 an ideal wideband photodiode transimpedance amplifier. Low-voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequency.

The key elements to a transimpedance design, as shown in Figure 40, are the expected diode capacitance (CD), which should include the parasitic input common mode and differential-mode input capacitance (4 pF + 5 pF for the OPA320); the desired transimpedance gain (RF); and the gain-bandwidth (GBW) for the OPA320 (20 MHz). With these three variables set, the feedback capacitor value (CF) can be set to control the frequency response. CF includes the stray capacitance of RF, which is 0.2 pF for a typical surface-mount resistor.

OPA320 OPA2320 OPA320S OPA2320S ai_trans_amp_dual_bos513.gif
CF is optional to prevent gain peaking. It includes the stray capacitance of RF.
Figure 40. Dual-Supply Transimpedance Amplifier

To achieve a maximally-flat, second-order Butterworth frequency response, the feedback pole should be set as shown in Equation 2.

Equation 2. OPA320 OPA2320 OPA320S OPA2320S q_fback_bos513.gif

Bandwidth is calculated by Equation 3.

Equation 3. OPA320 OPA2320 OPA320S OPA2320S q_bw_bos513.gif

For even higher transimpedance bandwidth, consider the high-speed CMOS OPA380 (90-MHz GBW), OPA354 (100-MHz GBW), OPA300 (180-MHz GBW), OPA355 (200-MHz GBW), or OPA656/57 (400-MHz GBW).

For single-supply applications, the +IN input can be biased with a positive dc voltage to allow the output to reach true zero when the photodiode is not exposed to any light, and respond without the added delay that results from coming out of the negative rail; this configuration is shown in Figure 41. This bias voltage also appears across the photodiode, providing a reverse bias for faster operation.

OPA320 OPA2320 OPA320S OPA2320S ai_trans_amp_single_bos513.gif
CF is optional to prevent gain peaking. It includes the stray capacitance of RF.
Figure 41. Single-Supply Transimpedance Amplifier

For additional information, see the Application Bulletin Compensate Transimpedance Amplifiers Intuitively (SBOA055), available for download at www.ti.com.

Optimizing the Transimpedance Circuit

To achieve the best performance, components should be selected according to the following guidelines:

  1. For lowest noise, select RF to create the total required gain. Using a lower value for RF and adding gain after the transimpedance amplifier generally produces poorer noise performance. The noise produced by RF increases with the square-root of RF, whereas the signal increases linearly. Therefore, signal-to-noise ratio improves when all the required gain is placed in the transimpedance stage.
  2. Minimize photodiode capacitance and stray capacitance at the summing junction (inverting input). This capacitance causes the voltage noise of the op amp to be amplified (increasing amplification at high frequency). Using a low-noise voltage source to reverse-bias a photodiode can significantly reduce its capacitance. Smaller photodiodes have lower capacitance. Use optics to concentrate light on a small photodiode.
  3. Noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor across the RF to limit bandwidth, even if not required for stability.
  4. Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit board carefully. A circuit board guard trace that encircles the summing junction and is driven at the same voltage can help control leakage.

For additional information, refer to the Application Bulletins Noise Analysis of FET Transimpedance Amplifiers (SBOA060), and Noise Analysis for High-Speed Op Amps (SBOA066), available for download at www.ti.com.

High-Impedance Sensor Interface

Many sensors have high source impedances that may range up to 10 MΩ, or even higher. The output signal of sensors often must be amplified or otherwise conditioned by means of an amplifier. The input bias current of this amplifier can load the sensor output and cause a voltage drop across the source resistance, as shown in Figure 42, where (VIN+ = VS – IBIAS × RS). The last term, IBIAS × RS, shows the voltage drop across RS. To prevent errors introduced to the system as a result of this voltage, an op amp with very low input bias current must be used with high impedance sensors. This low current keeps the error contribution by IBIAS × RS less than the input voltage noise of the amplifier, so that it does not become the dominant noise factor. The OPA320 series of op amps feature very low input bias current (typically 200 fA), and are therefore ideal choices for such applications.

OPA320 OPA2320 OPA320S OPA2320S ai_noise_ibias_bos513.gif Figure 42. Noise as a Result of IBIAS

Driving ADC'S

The OPA320 series op amps are well-suited for driving sampling analog-to-digital converters (ADC's) with sampling speeds up to 1 MSPS. The zero-crossover distortion input stage topology allows the OPA320 to drive ADC's without degradation of differential linearity and THD.

The OPA320 can be used to buffer the ADC switched input capacitance and resulting charge injection while providing signal gain. Figure 44 shows the OPA320 configured to drive the ADS8326.

OPA320 OPA2320 OPA320S OPA2320S ai_2opa_hifrq_cmr_bos513.gif Figure 43. Two Op Amp Instrumentation Amplifier With
Improved High-Frequency Common-Mode Rejection
OPA320 OPA2320 OPA320S OPA2320S ai_drv_ads8326_bos513.gif
Suggested value; may require adjustment based on specific application.
Single-supply applications lose a small number of ADC codes near ground as a result of op amp output swing limitation. If a negative power supply is available, this simple circuit creates a –0.3-V supply to allow output swing to true ground potential.
Figure 44. Driving the ADS8326

Active Filter

The OPA320 is well-suited for active filter applications that require a wide bandwidth, fast slew rate, low-noise, single-supply operational amplifier. Figure 45 shows a 500-kHz, second-order, low-pass filter using the multiple-feedback (MFB) topology. The components have been selected to provide a maximally-flat Butterworth response. Beyond the cutoff frequency, roll-off is –40 dB/dec. The Butterworth response is ideal for applications requiring predictable gain characteristics, such as the anti-aliasing filter used in front of an ADC.

One point to observe when considering the MFB filter is that the output is inverted, relative to the input. If this inversion is not required, or not desired, a noninverting output can be achieved through one of these options:

  1. Adding an inverting amplifier;
  2. Adding an additional second-order MFB stage; or
  3. Using a noninverting filter topology, such as the Sallen-Key (shown in Figure 46).

MFB and Sallen-Key, low-pass and high-pass filter synthesis is quickly accomplished using TI’s FilterPro™ program. This software is available as a free download at www.ti.com.

OPA320 OPA2320 OPA320S OPA2320S ai_2order_lopass_filt_bos513.gif Figure 45. Second-Order, Butterworth, 500-kHz, Low-Pass Filter
OPA320 OPA2320 OPA320S OPA2320S ai_sallen_key_bos513.gif Figure 46. OPA320 Configured as a Three-Pole, 20-kHz Sallen-Key Filter

Typical Application

OPA320 OPA2320 OPA320S OPA2320S Low_Pass_Filter_SBOS079.gif Figure 47. Second-Order, Low-Pass Filter Schematic

Design Requirements

  • Gain = 1 V/V
  • Low-pass cutoff frequency = 50 kHz
  • –40-db/dec filter response
  • Maintain less than 3-dB gain peaking in the gain versus frequency response

Detailed Design Procedure

The infinite-gain multiple-feedback circuit for a low-pass network function is shown in. Use Equation 4 to calculate the voltage transfer function.

Equation 4. OPA320 OPA2320 OPA320S OPA2320S App_EQ_1_SBOS165.gif

This circuit produces a signal inversion. For this circuit, the gain at DC and the lowpass cutoff frequency are calculated by Equation 5.

Equation 5. OPA320 OPA2320 OPA320S OPA2320S App_EQ_2_SBOS165.gif

Software tools are readily available to simplify filter design. WEBENCH® Filter Designer is a simple, powerful, and easy-to-use active filter design program. The WEBENCH® Filter Designer lets you create optimized filter designs using a selection of TI operational amplifiers and passive components from TI's vendor partners.

Available as a web-based tool from the WEBENCH Design Center, WEBENCH Filter Designer allows you to design, optimize, and simulate complete multistage active filter solutions within minutes.

Application Curve

OPA320 OPA2320 OPA320S OPA2320S D003_sbos079.gif Figure 48. OPA320 Second-Order, 50-kHz, Low-Pass Filter