The meaning of the parameters related to operational amplifiers that will be encountered in the future will be recorded here.
Recently, while using a PGA, I found that there is always a rectangular wave signal in the output when the PGA input is grounded. After amplification by 1000 times, it is very obvious, and I suspect that it is caused by interference from the power supply. At the beginning, 100uf and 0.1 capacitors were added to both the positive and negative power inputs, but the effect was not significant. Later, we planned to connect a resistor in series with the power input terminal. Initially, we chose 1k resistor, but after powering on, we found that the chip could not work at all. We measured the power supply voltage at both ends of the chip and found that it was only slightly above volts. At this point, I looked at the static current in the data manual and found that it was actually 5mA. The PGA is powered by 5V, and if the PGA works normally, the voltage division on the 1k resistor can reach 5V. So later, a 50 ohm resistor was used in combination with 100uf and 0.1uf to form a low-pass filter. This way, the chip worked normally and the output ripple was also much smaller.
When choosing an operational amplifier, one should know their design requirements and search for them in the operational amplifier parameter table. Generally speaking, the issues that need to be considered in design include 1 Selection of power supply voltage and mode for operational amplifiers; 2. Selection of operational amplifier packaging; 3. Operational amplifier feedback method, which is VFA (voltage feedback operational amplifier) or CFA (current feedback operational amplifier); 4. Operational amplifier bandwidth; 5. Selection of bias voltage and bias current; 6 temperature drift; 7. Pendulum rate; 8. Selection of input impedance for operational amplifiers; 9. Selection of output driving capability for operational amplifiers; 10. Static power consumption of operational amplifiers, i.e. selection of ICC current size; 11. Selection of operational amplifier noise; 12. Operational amplifier drives load stabilization time, etc.
Bias voltage and input bias current
In precision circuit design, bias voltage is a key factor. For 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 bias voltage drift of less than 200 μ V and input voltage noise of less than 6nV/√ Hz. The bias voltage drift with temperature variation is required to be less than 1 μ V/℃.
The indicator of low bias voltage is important in high gain circuit design, as amplifying the bias voltage may cause high voltage output and occupy a large part of the output swing. The temperature sensing and tension measurement circuit is an application example using precision amplifiers.
Low input bias current is sometimes necessary. The amplifier in the light receiving system must have low bias voltage and 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 operational amplifiers with the minimum input bias current.
Because I am currently using a photovoltaic system for data collection, I am particularly concerned with bias voltage and current during use. If there are other needs, more consideration should also be given to other parameters at this time.
1. Input Offset Voltage (VIO)
The input offset voltage is defined as the compensation voltage applied between the two input terminals when the output terminal voltage of the integrated operational amplifier is zero.
The input offset voltage actually reflects the symmetry of the internal circuit of the operational amplifier, and the better the symmetry, the smaller the input offset voltage. Input offset voltage is a very important indicator for operational amplifiers, especially for precision operational amplifiers or when used for DC amplification.
2. Input Offset Voltage Drift (VIO)
The temperature drift (also known as temperature coefficient) of input offset voltage is defined as the ratio of the change in input offset voltage to the change in temperature within a given temperature range.
This parameter is actually a supplement to the input offset voltage, which facilitates the calculation of the drift caused by temperature changes in the amplifier circuit within a given operating range. The input offset voltage temperature drift of general operational amplifiers is between ± 10~20 μ V/℃, while the input offset voltage temperature drift of precision operational amplifiers is less than ± 1 μ V/℃.
3. Input Bias Current IB
In the use of operational amplifiers, there may also be an input bias current IB, which refers to the DC current at the base of the input transistor of the first stage amplifier. This current ensures that the amplifier operates within a linear range, providing a DC operating point for the amplifier.
The input bias current is defined as the average bias current at the two input terminals of an operational amplifier when the output DC voltage is zero.
The input bias current has a significant impact on areas that require input impedance, such as high impedance signal amplification and integration circuits. The input bias current is related to the manufacturing process, and the input bias current for bipolar process (i.e. the standard silicon process mentioned above) is between ± 10nA and 1 μ A; For input stages using field-effect transistors, the input bias current is generally less than 1nA.
For bipolar operational amplifiers, the value has a high degree of variability, but is almost unaffected by temperature; For MOS type operational amplifiers, this value is the gate leakage current, which is small but greatly affected by temperature.
4. Input Offset Current
Input offset current refers to the error in bias current between two differential input terminals.
The input offset current is defined as the difference in bias current between the two input terminals of an operational amplifier when the output DC voltage is zero.
The input offset current also reflects the symmetry of the internal circuit of the operational amplifier, and the better the symmetry, the smaller the input offset current. Input offset current is a very important indicator for operational amplifiers, especially for precision operational amplifiers or when used for DC amplification. The input offset current is approximately one percent to one tenth of the input bias current. The input offset current has a significant impact on small signal precision amplification or DC amplification, especially when larger resistors are used outside the operational amplifier (such as 10k or more). The impact of input offset current on accuracy may exceed that of input offset voltage. The smaller the input offset current, the smaller the midpoint offset during DC amplification, and the easier it is to handle. So for precision operational amplifiers, it is an extremely important indicator.
5. Input impedance
(1) Differential input impedance
Differential input impedance is defined as the ratio of the voltage change at the two input terminals to the corresponding current change at the input terminals when the operational amplifier operates in the linear region. Differential input impedance includes input resistance and input capacitance, and only refers to input resistance at low frequencies.
(2) Common mode input impedance
The common mode input impedance is defined as the ratio of the change in common mode input voltage to the corresponding change in input current when the operational amplifier operates on an input signal (i.e. the same signal is input at both input terminals of the operational amplifier). At low frequencies, it manifests as common mode resistance.
6. Voltage gain
(1) Open Loop Voltage Gain
In the absence of negative feedback (open-loop condition), the amplification factor of an operational amplifier is called open-loop gain, denoted as AVOL, which is written as: Large Signal Voltage Gain。 The ideal value of AVOL is infinite, generally ranging from thousands to tens of thousands of times, and its representation can be expressed in dB and V/mV.
(2) Closed Loop Gain
As the name suggests, it is the amplification factor of an operational amplifier with feedback.
7. Output Voltage Swing
When the operational amplifier operates in the linear region, under a specified load, the maximum voltage amplitude that the operational amplifier can output when powered by the current power supply voltage.
8. Input voltage range
(1) Differential input voltage range
The maximum differential input voltage is defined as the maximum allowable input voltage difference between the two input terminals of the operational amplifier.
When the allowed input voltage difference between the two input terminals of the operational amplifier exceeds the maximum differential mode input voltage, it may cause damage to the operational amplifier input stage.
(2) Common Mode Input Voltage Range
The maximum common mode input voltage is defined as the common mode input voltage when the operational amplifier operates in the linear region and its common mode rejection ratio characteristics significantly deteriorate.
It is generally defined as the maximum common mode input voltage corresponding to a 6dB decrease in common mode rejection ratio. The maximum common mode input voltage limits the range of maximum common mode input voltage in the input signal, and this issue needs to be taken into account in circuit design in the presence of interference.
9. Common Mode Rejection Ratio
The common mode rejection ratio is defined as the ratio of the differential mode gain to the common mode gain of an operational amplifier when it operates in the linear region.
Common mode rejection ratio is an extremely important indicator that can suppress common mode interference signals. Due to the large common mode rejection ratio, the common mode rejection ratio of most operational amplifiers is generally tens of thousands of times or more, and it is not convenient to compare directly with numerical values. Therefore, decibel recording and comparison are generally used. The common mode rejection ratio of a typical operational amplifier is between 80 and 120dB.
10. Supply Voltage Rejection Ratio
The power supply voltage suppression ratio is defined as the ratio of the input offset voltage of the operational amplifier to the variation of the power supply voltage when the operational amplifier operates in the linear region.
The power supply voltage suppression ratio reflects the impact of power supply changes on the output of operational amplifiers. So when used for DC signal processing or small signal processing analog amplification, the power supply of the operational amplifier needs to be carefully and meticulously processed. Of course, operational amplifiers with high common mode rejection ratio can compensate for a portion of the power supply voltage rejection ratio. In addition, when using dual power supply, the power supply voltage rejection ratios of positive and negative power supplies may not be the same.
11. Static power consumption
The static power of an operational amplifier at a given power supply voltage is usually in an unloaded state.
Here is the concept of static current IQ, which refers to the current consumed by the operational amplifier during no-load operation. This is the minimum current consumption of the operational amplifier (excluding sleep mode)
12. Slew Rate
The conversion rate of an operational amplifier is defined as the rate at which a large signal (including a step signal) is input to the input of the operational amplifier under closed-loop conditions, and the output rise rate of the operational amplifier is measured from its output.
Due to the fact that the input stage of the operational amplifier is in a switching state during the conversion period, the feedback loop of the operational amplifier does not function, meaning that the conversion rate is independent of the closed-loop gain. Conversion rate is an important indicator for large signal processing. For general operational amplifiers, the conversion rate SR<=10V/μ s, while for high-speed operational amplifiers, the conversion rate SR>10V/μ s. The current high-speed operational amplifier has a maximum conversion rate SR of 6000V/μ s. This is used for selecting operational amplifiers in large signal processing.
13. Gain bandwidth
(1) Gain Bandwidth Product
Gain bandwidth product, GBP， The product of bandwidth and gain.
(2) Unit gain bandwidth
The bandwidth when the amplification factor of the operational amplifier is 1.
The concepts of unit gain bandwidth and bandwidth gain product are somewhat similar, but different. It should be noted that for voltage feedback type operational amplifiers, the gain bandwidth product is a constant, but not for current type operational amplifiers, because for current type operational amplifiers, bandwidth and gain are not linearly related.
14. Output impedance
The output impedance is defined as the ratio of the voltage change to the corresponding current change when a signal voltage is applied to the output terminal of an operational amplifier when it operates in the linear region. At low frequencies, it only refers to the output resistance of the operational amplifier. This parameter is tested in an open-loop state.
15. Equivalent Input Noise Voltage
The equivalent input noise voltage is defined as any AC irregular interference voltage generated at the output of an operational amplifier with good shielding and no signal input.
When this noise voltage is converted to the input terminal of the operational amplifier, it is called the operational amplifier input noise voltage (sometimes also expressed as noise current). For broadband noise, the effective value of input noise voltage for ordinary operational amplifiers is about 10-20 μ V.
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