How to choose the right op amp

It's no secret that the trend in the modern electronics industry is to integrate more functionality into the smallest possible form factor. Mobile phones are one such example. Many manufacturers today integrate MP3 players, digital cameras and even satellite TV functionality into mobile phones. This market has grown tremendously over the past few years and is still expanding rapidly.

These products typically have shorter design cycles and testing takes longer than the actual design (approximately 4 months for design and 6 months for testing). For this reason, designers must select components carefully to avoid repeated modifications and delays in the final product.

The following highlights some useful design techniques, brief calculations, and general evaluation methods to help designers better evaluate.

In portable electronics, designers use their expertise and best judgment to select devices based on a variety of factors (size, cost, and performance). But these factors often come with trade-offs, and designers must carefully select components based on the desired end product. Like almost every other industry, the portable market, especially the mobile phone market, typically offers both high-end (multifunctional) and low-end (cheap) products.

Figure 1: Using multiple op amps to reduce output noise.

Mobile phone motherboards include different components such as operational amplifiers, audio amplifiers and preamplifiers, data converters, and ASICs. Before selecting an op amp, the designer must consider packaging options and whether a smaller package will degrade performance. Although small packages are popular in portable products, small packages can cause troubles and problems for designers. Op amps in plastic packages, such as the SC70, often do not achieve the same performance as their SOIC or MSOP counterparts. In a microchip-scale package (CSP) (which is essentially a bare die), the input bias current can shift by hundreds of orders of magnitude when exposed to light. This form of packaging is also prone to cracking during assembly.

Which parameters are the most important?

In battery-powered applications—especially PDAs and mobile phones—op amps with good PSRR performance (~80dB) should be selected because the battery voltage will drop with interference. Also, be careful with high-gain configurations because noise coupling into the op amp will cause the noise level to rise. Resistor selection is also critical, as larger resistance values ​​produce higher noise. Can designers take advantage of 4? Estimate Johnson noise or resistor noise, where R is in K ohms, so a 100K ohm resistor produces approximately 40nV noise!

If multiple op amps are used, one way to reduce noise is to use the scheme shown in Figure 1. This method reduces the output noise by a factor, where n is the number of amplifiers used. For the LMV651, the output noise will be reduced to approximately 12nV/. In addition, the designer must consider limiting the bandwidth to minimize noise: the designer can reduce the noise by placing a small capacitor in parallel with the feedback resistor.

The choice of op amp also depends on other components. A common challenge designers face is choosing the right op amp for their analog-to-digital converter (ADC). Although there are many types of data converters on the market, the matching rules between op amps and analog-to-digital converters are different, and designers must carefully consider certain guidelines before making a choice.

Figure 2: A simple low-pass filter at the op amp output.

A brief glance at the data sheets for both devices will provide useful information, but it is not enough. First, pick an op amp and an analog-to-digital converter with the same supply voltage. Then choose an operational amplifier with a small THD+N. If you can't find distortion data, look at the output impedance: an op amp with smaller output impedance usually means smaller THD. Speed ​​is another parameter that must be considered, and although faster op amp speeds are comfortable to use, there are some trade-offs that must be considered, such as higher power and occasional instability.

Depending on the ADC chosen, the designer should choose an amplifier that is at least 50 times faster than the ADC sampling rate. The slew rate may also be a limiting factor and the designer can rely on 2? fVp is calculated, where f is the input signal frequency and Vp is the maximum output swing. For example, a 100mV input signal at 400kHz (gain of 10) requires an amplifier slew rate of at least 2.5V/µs.

Once these basic parameters are determined, designers must consider settling time, which can be misleading. Most manufacturers specify op amp settling times to be within 0.1% or 0.01% of a specific input voltage. If the design requires higher accuracy, such as 16 bits, then a parameter in the 0.0015% range of full scale is required. One way to solve this problem is to estimate the settling time of the op amp based on the accuracy of the analog-to-digital converter using the following equation:

Here, N is the number of bits and f is the open-loop bandwidth of the amplifier.

For example, for an operational amplifier with a gain of 10, such as the LMV651, when the accuracy is 12 bits, the settling time is approximately 1.4μs; when the accuracy is 16 bits, the settling time is 1.65μs. This formula is only an approximation and does not take into account stray capacitance, motherboard inductance, or the input capacitance of the analog-to-digital converter, all of which will affect settling time.

Before making the final choice, one of the most important indicators is the noise of the operational amplifier. A higher-noise amplifier will reduce the accuracy of the analog-to-digital converter and bring significant errors to the system. Before starting to calculate the total output noise of the circuit (which can be a very tedious task), it is best to make an estimate first. This way the designer knows whether he should continue to use the selected amplifier. This estimate involves the combined voltage noise of the op amp over the relevant bandwidth and the gain of the op amp configuration. We can express this formula as:

Here, NG is the noise gain, en is the voltage noise of the op amp, and BW is the closed-loop bandwidth.

In the circuit of Figure 2, a simple low-pass filter is used at the output. In this example, the output noise is the combined noise at this filter bandwidth (calculated as 1/2πRC). If a second-order filter is used, the bandwidth should be multiplied by a factor of 1.05.

Using the above formula and the LMV651 voltage noise density (17nV/), the total output RMS noise of the Figure 2 circuit at 100kHz bandwidth (filter bandwidth) is 53.7V. Once the total output noise is estimated, the designer can calculate the op amp's signal-to-noise ratio (SNR) using the following formula:

Here, VFS is the full-scale voltage range and Eout is the op amp noise calculated above. For example, a 2.5V signal produces a signal-to-noise ratio of 86.4dB.

The designer should then calculate the total SNR of the amplifier and analog-to-digital converter according to the following equation:

The SNR of ADC121S021 is 72.3dB. When ADC121S021 is paired with LMV651, the total SNR is 72.1dB. Ignoring harmonics, the designer can convert this SNR to an equivalent number of bits: ENOB = (SNR-1.76)/6.02, and then determine that only about 0.3dB is lost based on the equivalent number of bits, which is equivalent to a total accuracy error of 0.03% .

Since noise is the integrated noise at a specific bandwidth, it is obvious that the noise is also proportional to the bandwidth. In other words, reducing the bandwidth will reduce the noise; extending the bandwidth will increase the noise. If the decision is made to select a higher bandwidth filter, the designer should consider selecting a lower noise amplifier. For example, the 10MHz filter in the circuit of Figure 2 produces an overall SNR of less than 71dB, resulting in a loss of 0.5 bits. But when using the LMV791 (5.8nV/) with the same filter, the SNR increases to over 72dB. Designers can improve system accuracy simply by selecting lower noise op amps. But there are various trade-offs associated with this, such as power consumption and package size, that must be considered.

Other specifications to be considered

At this point, we have discussed the basic principles and rules for selecting components for your design, but there are other factors to consider. For example, for applications requiring higher accuracy, DC specifications such as input offset voltage and drift may be important.