SBOS073C September 1997 – August 2016 OPA2340 , OPA340 , OPA4340

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
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information

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.

The OPAx340 amplifier is a single-supply, CMOS operational amplifier with 5.5-MHz unity-gain bandwidth and supply current of 950 µA. Its performance is optimized for low-voltage (2.7 V to 5.5 V), single-supply applications, with its input common-mode voltage linear range extending 300 mV beyond the rails and the output voltage swing within 5 mV of either rail. The OPAx340 series features wide bandwidth and unity-gain stability with rail-to-rail input and output for increased dynamic range. Power-supply pins must be bypassed with 0.01-µF ceramic capacitors.

Figure 29 shows the OPA340 in a typical noninverting application with the input signal bandwidth limited by the input lowpass filter.

Equation 1 through Equation 2 show calculations for corner frequency and gain:

Equation 1.

Equation 2.

When receiving low-level signals, limiting the bandwidth of the incoming signals into the system is often required. The simplest way to establish this limited bandwidth is to place an RC filter at the noninverting terminal of the amplifier, as shown in Figure 29. If a steeper attenuation level is required, a two-pole or higher-order filter may be used.

The design goals for this circuit include these parameters:

- A noninverting gain of 10 V/V (20 dB)
- Design a single-pole response circuit with –3-dB rolloff at 15.9 kHz and 159 Hz
- Modify the design to increase attenuation level to –40 dB/decade (Sallen-Key Filter)

Use these design values:

- C
_{1}= 0 nF, 10 nF, 1 µF - R
_{1}= 1 kΩ - R
_{G}= 10 kΩ - R
_{F}= 90 kΩ

Figure 30 shows how the output voltage of OPA340 changes over frequency depending on the value of C_{1} with a constant R_{1} of 1 kΩ. Without any filtering of the input signal (C_{1} = 0), the –3-dB effective bandwidth is a function of the OPA340 unity-gain bandwidth and closed-loop gain, f_{(–3dB)} = UGBW/A_{CL}, where A_{CL} is closed-loop gain and UGBW denotes unity-gain bandwidth. Thus, for a closed-loop gain = 10, f_{(–3dB)} = 1 MHz/10 =100 kHz; see Figure 30.

To further limit the output bandwidth, an appropriate choice of C_{1} must be made: for C_{1} = 10 nF, = 15.9 kHz.

To further limit the bandwidth, a larger C_{1} must be used: choosing C_{1} = 1 µF,

= 159 Hz (see Figure 30).

If even more attenuation is required, a multiple pole filter is required. The Sallen-Key filter may be used for this task, as shown in Figure 31. For best results, the amplifier must have effective bandwidth that is at least 10 times higher than the filter cutoff frequency. Failure to follow this guideline results in a phase shift of the amplifier, which in turn leads to lower precision of the filter bandwidth. Additionally, to minimize the loading effect between multiple RC pairs on overall the filter cutoff frequency, choose R_{ } = 10 × R_{1} and C_{2} = C1/10; see Figure 32.

Equation 3 through Equation 5 show calculations for corner frequency and gain:

Equation 3.

Equation 4.

Equation 5.

Use these design values:

- C
_{1}= 10 nF and C_{2}= 1 nF - R
_{1}= 1 kΩ and R_{2}= 10 kΩ - R
_{G}= 10 kΩ - R
_{F}= 90 kΩ

Figure 32 shows the Sallen-Key filter second-order response for different RC values: for R and C values above, = 15.9 kHz.

To further limit the bandwidth, a larger RC value must be used: increasing C values 100 times, such as

C_{1} = 1 µF and C_{2} = 0.1 µF, with unchanged resistors, results in the second-order rolloff at = 159 Hz. See Figure 32.