In the design of analog circuits, we need to deal with various voltage and current small signals, at this time many engineers will subconsciously think of "co-direction operational amplifiers" and "reverse operational amplifiers", because in college "Mode Electricity" books teachers taught, so the memory is deep. However, the "differential amplifier circuit" is often forgotten and will not be used naturally, and it is a pity not to master the use of the "differential amplifier circuit", because it is too important in our electronic circuit design.
Let's first take a look at our commonly used "co-direction op amp" and "reverse op amp", as shown in Figure 1 and Figure 2, according to the definition of virtual short and false cut of "ideal op amp", the voltage gain formula of "co-direction op amp" and "reverse op amp" can be obtained through the Lekirhof voltage and current equation.
Figure 1 Circuit structure and gain calculation formula of a co-direction op amp
Figure 2 Reverse op amp circuit structure and gain calculation formula
The "co-directional amplification circuit" and "reverse amplification circuit" shown in Figures 1 and 2 have obvious drawbacks: the input resistance of the two inputs of the op amp is not equal, that is, the two ends of the op amp are asymmetrical, which will lead to a mismatch between the input bias currents at both ends of the op amp, resulting in an input offset current Ios and an input offset voltage Vos, which is ultimately reflected in the large noise voltage at the output of the op amp, reducing the amplification accuracy of the amplifier circuit. We can overcome the noise voltage effects caused by the asymmetry across the op amp by slightly changing the circuits shown in Figure 1 and Figure 2, as shown in Figure 3.
Figure 3 Symmetrical co-directional amplification circuit and reverse amplifier circuit at the input
In Figure 3, although the input resistance of the "co-directional amplifier circuit" and the "reverse amplifier circuit" is equal, the effect of the input offset current is reduced, but it is still limited in use. The voltage gain of the "co-amplification circuit" is greater than 1, and the signal cannot be reduced, which may cause the output voltage to exceed the limit and exceed the reference level of the back-end analog-to-digital converter (ADC) and cannot be converted; while the "reverse amplification circuit" will reverse the input signal, the positive voltage signal input will get a negative voltage output, and a dual power supply is required to supply power to the op amp, and the back-end analog-to-digital converter generally cannot handle the negative voltage signal. Therefore, to solve the above problems, it is necessary to use a differential discharge circuit, which can be signal amplified or signal reduction, and the standard differential amplifier circuit is shown in Figure 4.
Figure 4 Standard differential amplifier circuit
In the standard differential amplifier circuit in Figure 4, vref is the reference voltage to level up the entire input signal, so that even if the input signal source has a negative voltage, after the level is raised, there will be no negative voltage at the op amp output, which is convenient for back-end analog-to-digital conversion. In general, the selection of Vref needs to consider the analog reference level of the back-end ADC (that is, the highest voltage that the ADC can convert), if the analog reference level of the ADC is 5V, then we can set the reference voltage Vref to 2.5V, so that the input signal source as a whole is raised by 2.5V, so that the forward voltage of the input signal source is changed between 2.5V and 5V after the op amp processing, and the negative voltage of the input signal source is changed between 0V and 2.5V after the op amp processing. Because the op amp outputs are all forward voltages, the power supply of the op amp does not need to use dual power supplies, only a single power supply can be used.
Here's a case I've done:
In the power system or electric vehicle application field, it is often necessary to measure the charge and discharge current of the battery pack (lead-acid battery or lithium battery) in the energy storage system, obviously the current is directional, if the charge is defined as positive, then the discharge is negative, assuming that the charge and discharge current of the battery pack range is -200A ~ 200A.
The CPU was selected for the Freescale processor MPC5644A, which can be sampled using a 12-bit analog-to-digital converter (ADC) inside the CPU, which supplies 5V, but whose internal ADC has a dedicated analog reference level pin (VRH, VRL), as shown in Figure 5.
Figure 5 Analog reference level pinout of the MPC5644A microprocessor
(1) First select the Hall current sensor model: according to the current measurement range to select Feixuan SZ128E2-200A, the specification parameters as shown in Figure 6, the Hall using dual power supply ± 12V or ± 15V power supply, in the -200A ~ 200A measurement range, hall output voltage range corresponds to -4V ~ 4V;
Fig. 6 Hall current sensor SZ128E2-200A specifications
(2) Then design the sampling detection circuit of the Hall current sensor and determine the CPU analog reference voltage: the op amp uses a single-supply 5V power supply, selects the differential amplifier circuit to reduce the Hall voltage output signal, and then raises the overall through the reference level VREF, so that the op amp outputs a forward voltage signal. According to the Hall output voltage range of -4V to 4V, we can set the CPU's internal ADC analog reference level VRH=4V, obtained by using the TL431 design, and obtain the op amp reference level VREF=2V through high-precision resistor divider (in general, the op amp reference level is selected as half of the ADC analog reference level), as shown in Figures 7 and 8;
Figure 7 Sampling detection circuit of a Hall current sensor
Figure 8 ADC analog reference level VRH and op amp reference level VREF are designed using the TL431
(3) Calculate the voltage signal into the ADC, that is, the voltage output after the op amp is conditioned: According to the standard differential op amp gain calculation formula shown in Figure 4, the output of Hall after the op amp is obtained: Vo=-(R22/R20) * Vct+Vref=-(4.7/10)*Vct+2; Therefore, when the Hall charge current reaches a maximum of 200A, Vct=4V, then Vo=0.12V is calculated; when the Hall discharge current reaches a maximum of 200A, Vct=-4V is calculated. Then the Vo=3.88V is calculated; it can be seen that the output of Vo is between 0.12V and 3.88V, and this range segment is within the range of VRH=4V of the ADC analog reference level, and the limit will not be exceeded. The emitter follower in Figure 7 is used to isolate and improve the reference level VREF with load capacity, otherwise the positive resistors R21 and R23 of the Op Amp U8 in Figure 7 will have an effect on the voltage division resistance of the VREF front end, so that the reference level VREF obtained by the voltage division is incorrect.
In summary: for the input signal source has a positive and negative direction, when using the op amp, it is necessary to raise the voltage through the reference level, so that the signal into the ADC can be a forward voltage, we should try to use a differential op amp when using the op amp, the differential op amp common mode input voltage is the smallest (the final common mode interference signal presented at the output of the op amp will be small), can well suppress the input common mode interference signal and clutter, and because the input terminal is symmetrical (the op amp positive and negative input resistance is equal), The overall input offset current and offset voltage of the circuit are also much smaller than those of the co-direction and reverse amplification circuits. (End of article)
If the article helps your technical progress and ability to improve, please help "forward", and click "follow", welcome to leave a message in the comment area to discuss, exchange ideas, we learn and progress together, thank you!