The meaning of the parameters relating to the op amp that will be touched later in the use of the op amp is noted here. Recently, when using a PGA, when the PGA input is grounded, it is found that there is always a rectangular wave signal at the output. After 1000 times of amplification, it is very obvious. It is suspected that the power supply causes interference. At the beginning, 100uf and 0.1 capacitors were added to the input positive and negative power supply, but the effect was not obvious. Later, a resistor was connected in series with the power input terminal. At the beginning, the resistor was selected to be 1k, but after power-on, the chip was found to be Can not work, measuring the power supply voltage at both ends of the chip found only a little more v. At this time, I looked at the quiescent current of the data sheet and found that it was 5mA. Then the PGA is powered by 5v. If the PGA works normally, the voltage divider on the 1k resistor can reach 5v. So later I used a 50 ohm resistor with 100uf and 0.1uf to form a low-pass filter, so that the chip works fine, and then the output ripple is much smaller.
When choosing an op amp, you should know what your design needs are and look it up in the op amp parameter table.
Generally, the issues that need to be considered in the design include
1. The size and mode of the op amp supply voltage;
2. Op amp package selection;
3. The op amp feedback mode, that is, VFA (voltage feedback op amp) or CFA (current feedback op amp);
4. Op amp bandwidth;
5. Bias voltage and bias t current selection;
6 temperature drift;
7. Slew rate;
8. Op amp input impedance selection;
9. Op amp output drive capability size selection;
10. Op amp static power consumption, that is, ICC current size selection;
11. Op amp noise selection;
12. Op amp drive load stabilization time and so on.
Bias Voltage and Input Bias Current Bias voltage is a key factor in precision circuit design. For those parameters that are often overlooked, such as bias voltage drift and voltage noise that vary with temperature, they must also be measured. Accurate amplifiers require a bias voltage drift of less than 200μV and an input voltage noise of less than 6nV/√Hz. The bias voltage drift with temperature is required to be less than 1μV/°C. The low bias voltage specification is important in high gain circuit design because the bias voltage is amplified to cause a large voltage output and will occupy a large portion of the output swing. Temperature sensing and tension measurement circuits are examples of applications that utilize precision amplifiers. Low input bias currents are sometimes required. The amplifier in the light receiving system must have a low bias voltage and a low input bias current. For example, the dark current of a photodiode is on the order of pA, so the amplifier must have a smaller input bias current. CMOS and JFET input amplifiers are currently available as op amps with minimal input bias current. Because I am using a photocell for acquisition system, I focus on the bias voltage and current during use. If there are other needs, then you should consider more of the other parameters.
1. Input Offset Voltage The input offset voltage is defined as the compensation voltage applied between the two inputs when the voltage at the output of the integrated op amp is zero. The input offset voltage actually reflects the circuit symmetry inside the op amp. The better the symmetry, the smaller the input offset voltage. Input offset voltage is a very important indicator of op amps, especially for precision op amps or for DC amplification.
2. Temperature drift of the input offset voltage (Variation Offset Voltage Drift) The temperature drift of the input offset voltage (also called temperature coefficient) is defined as the ratio of the change of the input offset voltage to the temperature change within a given temperature range. This parameter is actually a supplement to the input offset voltage, which is convenient for calculating the drift of the amplifier circuit due to temperature changes within a given operating range. Generally, the input offset voltage of the op amp drifts between ±10~20μV/°C, and the input offset voltage drift of the precision op amp is less than ±1μV/°C.
3. Input Bias Current IB (Input Bias Current) may also encounter an input bias current IB in the operational amplifier. The input bias current refers to the base DC current of the input amplifier of the first stage amplifier. This current ensures that the amplifier operates in a linear range and provides a DC operating point for the amplifier. The input bias current is defined as the average of the bias current at the two inputs when the op amp's output DC voltage is zero. The input bias current has a large influence on where high-impedance signal amplification, integration circuits, etc. require input impedance. The input bias current has a certain relationship with the manufacturing process. The input bias current of the bipolar process (ie, the standard silicon process described above) is between ±10nA and 1μA; the field effect transistor is used as the input stage, and the input bias current is used. Generally less than 1nA. For bipolar op amps, this value is highly discrete, but is almost unaffected by temperature; for MOS op amps, this value is the gate leakage current, which is small, but is greatly affected by temperature.
4. Input Offset Current Input offset current is the error of the bias current of the two differential inputs. The input offset current is defined as the difference in bias current between the two inputs when the output DC voltage of the op amp is zero. The input offset current also reflects the circuit symmetry inside the op amp. The better the symmetry, the smaller the input offset current. Input offset current is a very important indicator of op amps, especially for precision op amps or for DC amplification. The input offset current is approximately one-tenth to one-tenth the input bias current. The input offset current has a significant impact on small signal precision amplification or DC amplification. Especially when the op amp uses a large resistor (for example, 10k or more), the input offset current may affect the accuracy more than the input offset voltage. influences. The smaller the input offset current is, the smaller the intermediate zero offset is during DC amplification, and the easier it is to handle. So it is an extremely important indicator for precision op amps.
5, input impedance (1) differential mode input impedance differential mode input impedance is defined as the ratio of the voltage change between the two input terminals and the corresponding input current change when the op amp operates in the linear region. The differential mode input impedance includes the input resistance and the input capacitance, and only refers to the input resistance at low frequencies. (2) Common mode input impedance The common mode input impedance is defined as the ratio of the change in the common mode input voltage to the corresponding change in the input current when the op amp is operating on the input signal (ie, the same signal is input to the two inputs of the op amp). . At low frequencies, it behaves as a common mode resistor.
6, voltage gain (1) open-loop voltage gain (Open-Loop Gain) without negative feedback (open loop condition), the amplifier's amplification is called open-loop gain, recorded as AVOL, and some datasheet Written as: Large Signal Voltage Gain. The ideal value of AVOL is infinite, generally about thousands to tens of thousands of times, and its representation uses dB and V/mV. (2) Closed-Loop Gain As the name suggests, it is the amplification factor of the op amp when there is feedback.
7. Output Voltage Swing When the op amp is operating in the linear region, the maximum voltage amplitude that the op amp can output when the current supply voltage is supplied under the specified load.
8. Input voltage range (1) Differential mode input voltage range The maximum differential mode input voltage is defined as the maximum input voltage difference allowed by the two inputs of the op amp. When the input voltage difference allowed by the two inputs of the op amp exceeds the maximum differential mode input voltage, the op amp input stage may be damaged. (2) Common Mode Input Voltage Range The maximum common mode input voltage is defined as the common mode input voltage when the op amp's common mode rejection ratio is significantly degraded when the op amp operates in the linear region. . It is generally defined as the common mode input voltage as the maximum common mode input voltage when the common mode rejection ratio is reduced by 6dB. The maximum common-mode input voltage limits the maximum common-mode input voltage range in the input signal. In the presence of interference, this problem needs to be addressed in circuit design.
9. Common Mode Rejection Ratio The common mode rejection ratio is defined as the ratio of the op amp's differential mode gain to the common mode gain when the op amp is operating in the linear region. The common mode rejection ratio is an extremely important indicator that can suppress common mode interference signals. Since the common mode rejection ratio is very large, the common mode rejection ratio of most op amps is generally tens of thousands of times or more, and the numerical values ​​directly indicate inconvenient comparison, so the decibel method is generally used for recording and comparison. The common mode rejection ratio of a general op amp is between 80 and 120 dB.
10, Supply Voltage Rejection Ratio (Supply Voltage Rejection Ratio) The supply voltage rejection ratio is defined as the ratio of the op amp input offset voltage to the supply voltage when the op amp is operating in the linear region. The supply voltage rejection ratio reflects the effect of power supply variations on the op amp output. Therefore, when used for DC signal processing or small signal processing analog amplification, the power supply of the op amp needs to be carefully and carefully processed. Of course, an op amp with a higher common-mode rejection ratio can compensate for a part of the power supply voltage rejection ratio. In addition, when the dual power supply is used, the power supply voltage rejection ratio of the positive and negative power supplies may be different.
11. Static power The static power that is placed at a given supply voltage, usually under no load. There will be a concept of quiescent current IQ. The quiescent current actually refers to the current that it consumes when it is operated at no load. This is the minimum current consumption of the op amp (excluding sleep).
12. Slew Rate The op amp slew rate is defined as the input of a large signal (including a step signal) to the input of the op amp and the output of the op amp from the op amp. The output rise rate. Since the input stage of the op amp is in the switching state during the conversion, the feedback loop of the op amp does not work, that is, the slew rate is independent of the closed loop gain. The slew rate is an important indicator for large-signal processing. For a typical op amp slew rate SR<=10V/μs, the slew rate of the high-speed op amp is SR>10V/μs. The current high-speed op amps have a maximum slew rate of SR of 6000V/μs. This is used for op amp selection in large signal processing.
13. Gain Bandwidth (1) Gain Bandwidth Product Gain Bandwidth Product, GBP, Product of Bandwidth and Gain. (2) The bandwidth of the unity gain bandwidth operational amplifier when the amplification factor is 1. The two concepts of unity gain bandwidth and bandwidth gain product are somewhat similar but different. It should be noted here that the gain-bandwidth product is a constant for voltage feedback op amps, but not for current-mode op amps, because for current-mode op amps, bandwidth and gain are not linear. relationship.
14. Output Impedance The output impedance is defined as the ratio of the voltage change to the corresponding current change when the op amp is operating in the linear region at the output of the op amp. At low frequencies, it only refers to the output resistance of the op amp. This parameter is tested in an open loop state.
15. Equivalent Input Noise Voltage The equivalent input noise voltage is defined as any unbalanced interference voltage generated by an op amp with good shielding and no signal input at its output. When this noise voltage is converted to the op amp input, it is called the op amp input noise voltage (sometimes also expressed by the noise current). For wideband noise, the input noise voltage of a normal op amp is about 10~20μV.
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