The book continues above, we talked about the first parameter - the supply voltage Vss. This article will supplement the remaining parameters. Just to emphasize: an op amp needs at least five pins.
1. Short and false breaks
The above figure is a simplified diagram of the operation. First, let's review the basic methods of analyzing operations - virtual short and virtual break.
The symbols below the specification: the voltage at the reverse input terminal is V-, the current is I-, the voltage at the positive input terminal is V+, I+
Virtual short: V+=V-
Virtual break: I+=I-=0, we can understand this from the input impedance parameters of the ideal op amp, because the input impedance is infinite, when the limited voltage is added, I=U/R shows that I can be infinitely close to 0.
2. Power supply voltage Vss
Review the first parameter above - the supply voltage Vss.
In actual use, some op amps can be used with a single power supply or with a dual power supply, which is weighed according to the Datasheet and the actual manufacturer.
But there are a few basic principles: 1. For op amps powered by dual power supplies, the positive and negative power supplies should be connected at the same time as possible, otherwise the op amp will be easily damaged.
2 Do not connect live wires, pull out wires or change wires, etc.
for example:
OPA228.
By consulting the datasheet of OPA228, it can be seen that its Vss=plus or minus 20, and the maximum single power supply is 40V. Then Vcc+ and Vcc- can be less than or equal to 18V when the operational amplifier supplies power. When powered by a single power supply, Vcc is less than 35V, Vcc-=GND. There is a margin in actual use.
2. Input bias current IB (B is the subscript, it is not easy to edit)
Due to the virtual break and virtual short of the ideal op amp, the current flowing into the inverting input terminal and the non-inverting input terminal is zero. In practical applications, neither of them is zero. At this time, the input bias current IB (Input Bias Current) = [(Ib+)+(Ib-)]
IOS is defined as the difference between the two currents. IOS=(Ib+)-(Ib-). The smaller this parameter is in practice, the better.
For example: Ib+=7pA, Ib-=3pA. Then IB=(7+3)/2pA=5pA, IOS=7pA-3pA=4pA
Ib+ and Ib- can be detected by connecting ammeters at both ends of the reverse input terminal and the same input terminal respectively.
OPA227
It can be found in the datasheet of OPA227 that its IB=plus or minus 2.5 (TYP), and the unit of plus or minus 10 (MAX) is nA.
There are fA-level operational amplifiers on the market, specifically LMC6001, LMC6042A, LMC6062A, ICH8500A, AD549LH, OPA128LM, etc. Low IB operational amplifiers are generally used for precision current detection and other occasions that require low IB
You can refer to the book "Illustrated DIY 1pA Ultra-micro Current Tester"
Use a photodiode to detect the intensity of light, the photodiode current is very small, Rf needs to use a large resistance (GΩ or TΩ level resistance) to extract the weak current, and because its current is very weak, in order to prevent the power supply of Vcc and -Vcc The induced electromotive force generated by the power supply at the inverting input terminal will affect the detection current, and the GUARD RING (protection ring) technology can be used.
The process of the operational amplifier determines Vos and Ib, here we roughly talk about:
1. Bipolar: low input impedance, Ib=1-100nA, Vos=10-1000uV, as low as 0.1uV/℃, low voltage noise
2. JFET: High input impedance, Ib=10-100pA, but the disadvantage is that the temperature drift is large, which doubles every ten degrees Celsius. Vos=0.1-5mV
3. CMOS: high input impedance, low offset; rail-to-rail output capability; low power consumption; Iq can be as low as 700nA
TI has its own process: DiFET, extremely high input impedance, Ib as low as 3fA, low offset; extremely low current and voltage noise; best DC accuracy.
The above processes can be combined with each other as follows:
1. BiFET: Bipolar+JEFT: FET input, Bipolar output
2. BiCMOS: Bipolar+CMOS, common in analog-digital hybrid circuits, such as power chips.
For Vos and Ib, craftsmanship is key.
Pay special attention to these two parameters in places with high precision, and don't pay much attention to them in general use.
3. Noise Vn
Noise is inherent to the device and is random, and we cannot eliminate it.
Of course, in practical application, it is definitely hoped that the smaller the better.
When amplifying small signals, low-noise op amps must be used.
The unit of noise is: nV/root Hz@1KHz, the noise at different frequencies will be marked in the datasheet, @1KHz is a typical one.
For example, the Vn of OPA211 is 1.1nV/root Hz@1KHz
Noise can be divided into two types in op amps, one is white noise and the other is 1/f noise. White noise has nothing to do with frequency, 1/f is related to frequency, so there is a saying @1KHz
4. Quiescent current Iq
The standby current of the op amp under no load or the current when the quiescent operating point is established, the smaller the better.
In the case of low power consumption, special attention should be paid to this indicator. No requirement unless otherwise specified.
Such as OPA369, Iq of 1.1uA; TLV2401, 0.95uA.
Generally, other indicators of low-power op amps will be relatively poor.
OPA227
OPA188
OPA333
OPA211
5. Input offset voltage Vos
Vos (Input Offset Voltage) input offset voltage----It can be seen from the virtual short that the voltage of the same direction end and the reverse end are equal, but due to the mismatch between the same direction end and the reverse end, the inherent voltage difference is generated, which is called input Offset voltage, generally Vos is about (1-10) mV, high-quality operational amplifier Vos is below 1mv.
The calculation formula of Vos is: Vos=(V+)-(V-)
Vos, like Ib, the smaller the better.
OPA227
OPA188
OPA333
OPA211
OPA188>OPA277>OPA211
How to eliminate the input offset voltage Vos? In some op amps, there are extra pins, and we can adjust the resistance by sliding the rheostat to make Vos close to zero. (Applicable to the single op amp with its own zero-adjustment pin, follow the method given in the Datasheet to zero-adjust)
TI's zero adjustment pins are generally pins 1 and 8.
ADI Company: Generally, a sliding rheostat is added to pin 1 and pin 5 for zero adjustment.
Since the multi-channel op amp does not have a special zeroing pin, it can be zeroed by an adder. For the specific implementation, see "OP Amplifying Circuit Design" P34-P35.
6. Input offset voltage temperature drift dVos/dT
This parameter refers to the temperature coefficient of Vos within the specified operating range, and is an important indicator to measure the influence of the temperature of the operational amplifier.
This parameter can be checked or not, depending on the specific application scenario.
7. Gain Bandwidth Product (GBW is also called GBP)
When the op amp is used in a small signal (Vpp peak-to-peak <1V signal) environment, the bandwidth and gain product of the voltage feedback op amp is a constant value, which is equal to the gain bandwidth product.
Gain-bandwidth product and slew rate (SR) are the two most important indicators for selecting an op amp, and these two indicators are not separated. (In front of these two parameters is the power supply voltage Vss, the power supply voltage parameters must be determined first)
GBW is greater than or equal to (10-100)*Gain*f (Gain is the amplification factor, and the unit of f is brought in by MHz)
Assumption: to amplify a signal with a frequency of 1MHz six times, how much GBW should be used for the op amp?
GBW greater than or equal to (10-100)*6*1MHz=60-600MHz
Eight, the voltage conversion rate (Slew Rate, SR) referred to as the slew rate
It is defined as the amplitude of increasing the voltage within 1us or 1ns. Intuitively, it is the time required to increase the square wave voltage from the valley to the peak. The units are V/s, V/ms, V/us and V /ns Four kinds.
Specific calculation formula: SR is greater than or equal to 2*Π*f*Vom
Assumption: A sine wave with a maximum voltage of 1V and a frequency of 1MHz is amplified by six times, what size SR op amp should be used?
SR is greater than or equal to 2*3.14*1*6=37.68
OPA227
OPA228
OPA188
OPA 333
If the selected operational amplifier GBW and SR are not enough, the waveform will be distorted, and the sine wave should be output, but the triangular wave will be output
solution:
1. Replace the op amp with GBW and SR that meet the requirements
2. Using multi-stage amplification
3. Use current feedback op amp, which has higher GBW and SR, but it is easy to oscillate. The circuit should be designed in strict accordance with the requirements on the Datasheet.