Application of MAX4145 in Pseudo-Random Code Generator Circuit

1. Introduction  
With the rapid development of VLSI technology,

microprocessor technology and the application of some new components, spread spectrum technology has been widely used in various aspects of communication. Figure 1 shows a principle block diagram of a spread spectrum communication system. 

In general, the transmitter and receiver in the spread spectrum communication system must know a preset spread spectrum code in advance. This spread spectrum code is actually a pseudo-random digital sequence that is long enough and as close to noise as possible. The system can obtain a synchronization accuracy of half the code width through the capture and correlation of the pseudo-random code. In this way, the quality of the pseudo-random code and the accuracy of tracking and synchronization have a direct impact on the communication quality. Therefore, it is of great significance to design a high common mode rejection ratio and low noise preamplifier with excellent performance for spread spectrum communication systems. The differential amplifier MAX4145 chip of MAXIM is widely used in communication system design with its outstanding performance, and its indicators can fully meet the requirements of the pseudo-code generation circuit in the spread spectrum communication system. The MAX4145 series chips have high speed, low distortion, wide bandwidth and high common mode rejection ratio. They are ideal devices for differential circuits in high-speed data transmission systems. Therefore, they can be widely used in differential signal to single-ended signal conversion circuits, twisted pair and coaxial line conversion equipment, high-speed differential signal receiving circuits, high-speed amplification equipment, data acquisition equipment and medical equipment.

2 Working principle and performance characteristics of MAX4145  
2.1 Working principle of MAX4145



MAX4145 works in differential mode. It has the characteristics of small signal swing, few even harmonic components, and strong anti-interference ability to noise. Compared with the single-ended input mode, MAX4145 can provide better harmonic distortion (THD) and spurious free dynamic range (SFDR), so it has a higher common mode rejection ratio (CMRR).  

MAX4145 uses three op amp combination technology to complete the three functions of differential input, gain amplification and signal output. Its internal structure is shown in Figure 2. Among them, op amps A1 and A2 complete the differential input and gain amplification functions, and op amp A3 mainly performs signal output and impedance matching. 

In addition to the characteristics of large input impedance, the common-mode gain offset and drift errors of the front stage of MAX4145 can offset each other, and can suppress the common-mode signal of the back stage. At the same time, the double-ended signal can be converted into a single-ended output to meet the needs of the grounded load. In addition to the three op amps, MAX4145 also includes an output short-circuit self-protection circuit and an input protection circuit, thereby increasing the chip's anti-destruction capability. The gain can be set within the range of +1V/V to +10V/V through an external resistor RG. When RG is connected between pins RG- and RG+ (see Figure 2), the gain is calculated as follows:  

G = AV = 1 + (1.4 kΩ/RG)  
The common-mode rejection ratio is a parameter that measures the differential amplifier's ability to suppress common-mode signals. The larger the value of this parameter, the stronger the suppression ability.  

2.2 Performance indicators of MAX4145  
The main performance parameters of MAX4145 are as follows:  
●Adjustable gain range is +1V/V～+10V/V;  
●-3dB bandwidth is 180MHz (ＶＯＵＴ≤０.1VRＭＳ, ＡＶ＝１Ｖ／Ｖ);  
●Slew rate SR＝600V／μs (－２Ｖ≤ＶＯＵＴ≤＋２Ｖ);  
●Common mode rejection ratio CMRR＝75dB (f＝１０MHz);  
●Spurious free dynamic range SFDR＝－９2dBc (f＝１０ｋＨｚ);  
●Noise is 3.8nV/√Ｈｚ (G＝＋１０V／Ｖ);  
●Settling time ts＝２０ns (－２Ｖ≤ＶＯＵＴ≤＋２Ｖ, to ０.１％);  
●Power-down mode current is 800μA.

3 MAX4145 application points



MAX4145 can be operated in power-down mode by setting SHDN high, and the output is in high impedance state. 

Differential mode usually requires symmetrical driving of IN- and IN+, that is, after the two input signals are connected to the driving circuits of IN- and IN+, their phases must be consistent and their common-mode gain errors must be reduced as much as possible.  

In ordinary applications, when REF is grounded, SENCE can be connected to OUT. In some applications with long signal transmission distances, SENCE and OUT can be connected to the load at the same time, which can compensate for distance loss and reduce voltage errors. In order to reduce the output gain error and increase the frequency response, the capacitance and impedance of the SENCE end should be minimized during design. At the same time, the matching problem of REF and SENCE at the output end is also critical, because the mismatch between REF and SENCE ends will cause common-mode gain loss. 

Under normal use conditions, when the termination impedance is a non-capacitive load, the MAX4145 has the best AC performance. Generally, when the load capacitance does not exceed 25pF, the output voltage will not oscillate, but it will have a certain impact on the frequency response. Therefore, if the load capacitance is too large, the output will produce ringing. In order to drive a load with a large capacitance and reduce signal ringing, an isolation resistor can be added between the amplifier output and the load. The isolation resistor value can be determined by the signal frequency and the load capacitance. At this time, the bandwidth will be determined by the RC loop composed of the isolation resistor and the load capacitance. Therefore, increasing the load capacitance will reduce the signal bandwidth of the entire circuit, while the isolation resistor will reduce the voltage distributed to the load. [page]



4 Application in pseudo-random code generation circuit  

4.1 Pseudo-random code generation circuit  
Pseudo-random code sequences can generally be generated using a shift register network, which consists of an R-level series dual-state device shift pulse generator and a modulo-2 adder. Figure 3 shows a simple schematic diagram of a four-stage shift register network that can generate a pseudo-random code with a code length of 15. 

FPGA can be used to realize the shift register network to generate pseudo-random code signals, and realize functions such as logic control and clock distribution. There are two ways to process the TTL signal output by FPGA: one is to send it directly to the op amp for signal conditioning and output; the other is to output the TTL after D/A conversion and signal conditioning. After analysis and actual testing, the author found that the output pseudo-code quality is poor due to the serious phase jitter of the signal output by FPGA, which may even cause unstable signal edges, and there are serious parasitic signals; if it is conditioned and output after D/A conversion, this effect will be weakened and the signal quality will be improved. Therefore, the second method is more desirable. In practical applications, the author chooses this method for circuit design and selects the differential current output type D/A to directly output after amplification by MAX4145. The  

principle block diagram of the pseudo-random code generation circuit based on MAX4145 is shown in Figure 4. When the pseudo-random code generation circuit is working, the system can distribute the pseudo-code data to the FPGA through the parallel port, and the FPGA can also generate pseudo-code signals independently. At the same time, the FPGA completes signal processing, clock distribution, code synchronization generation, and waveform storage. The main function of MAX4145 is to complete the conversion and amplification from differential to single-ended output.



    4.2 MAX4145 application circuit design 

According to the system's demand for pseudo-random codes, the MAX4145 application circuit design is shown in Figure 5. In the figure, the input signals IN+ and IN- are converted by the upper D/A and then sent to the MAX4145 through the matching circuit. In the output circuit, REF is grounded, SENCE and OUT are connected, and the gain of the circuit is approximately equal to 4. 

4.3 Result measurement and analysis  
For pseudo-random codes, the main focus is usually on their overshoot and edge rise time. The author measured the input differential signal and output single-ended signal of the MAX4145 in the system, and the measurement results are listed in Table 1. Table 1 Measurement items

of MAX4145 signals in pseudo-random code system   Voltage (V) Positive phase overshoot (%) Rise time (ns) Input IN+ IN- IN+ IN- IN+ IN- 0.42 -0.42 28.57 26.67 10.55 11.50 Output -1.70～+1.72 6.92 10.53 It can be seen from the data in Table 1 that the use of MAX4145 can greatly reduce the overshoot of the input signal, and the signal edge rise time is also improved, which can generate pseudo-random codes with relatively ideal related characteristics. In actual spread spectrum communication systems, these improvements and enhancements will be more conducive to signal recovery and demodulation, thereby improving system performance.