Design of preamplifier circuit for portable multifunctional digital analyzer based on AD8260

0 Introduction
Oscilloscopes and frequency meters are commonly used instruments for electronic measurement. The design of their front-end circuits plays a key role in the overall performance of the instrument. This article mainly introduces the design method of a digital analyzer front-end module based on AD8260. AD8260 is a high-current driver and low-noise digital programmable variable gain amplifier (DGA) produced by AD. The gain adjustment range of this device is -6~+24 dB, and the -3dB bandwidth of the adjustable gain is 230 MHz. It can be powered by a single power supply of 3.3 V, 5 V or +/-5 V dual power supply. It is mainly used in digital control automatic gain system, transceiver signal processing and other fields. This design mainly uses its digital control automatic gain function.

1 Design Principle
The digital analyzer includes two functions: an oscilloscope and a frequency meter. The frequency meter function is divided into two parts: high frequency band and low frequency band. The frequency signal below 20 MHz is measured by the oscilloscope after sampling; the frequency signal above 20 MHz is realized by the frequency meter through an equal precision algorithm. In order to improve the measurement accuracy, this design uses a high-stability constant temperature crystal oscillator to provide the clock source for the system, and provides each module with a clock signal through the built-in phase-locked loop of the FP-GA. For the realization of the 100 MHz bandwidth oscilloscope function, if it is realized by ultra-high-speed sampling of a single chip, the sampled high-speed signal must be stored. This method is not easy to implement and has high requirements on the process of the circuit. In order to avoid this situation. This design refers to the design of TEKtronix's 100 MHz oscilloscope, and thus uses 3 ADCs to realize the time-sharing sampling of the input signal, and the DSP processes it after storing it separately.
This article mainly introduces the design of the front-end module of the digital analyzer. Since the input signal amplitude range of the oscilloscope is generally +/-5Vpp, and the input signal amplitude of AD9433 (AD sampling chip) is required to be no more than 2Vpp, adding a two-stage amplitude adjustment circuit of signal attenuation and signal gain at the front end of the acquisition circuit can ensure the correctness of the measurement and improve the accuracy of the measurement. Unlike NI's digital analyzer, the frequency meter function of this digital analyzer is not realized by digital signal analysis after ultra-high-speed sampling, but is realized based on the principle of equal-precision frequency measurement, which can greatly reduce the cost of the instrument. At the same time, considering the portability of the instrument and the application habits of engineers, this instrument only provides a single-channel input and does not design an external trigger channel, thereby simplifying the size of the instrument and reducing the difficulty of design. The principle block diagram of the front-end module of the digital analyzer is shown in Figure 1.

After the input signal of this system passes through the voltage protection circuit and AC/DC coupling selection, it will enter the high-resistance attenuation network, and realize the high-resistance attenuation of the input signal of 1:1 and 1:10 through the external control signal. The attenuated signal enters the AD8260 for program-controlled gain after differential transformation. The signal after program-controlled gain directly enters the AD9433 for high-speed sampling (here the signal amplitude after gain is required to reach the full-scale input of AD9433 as much as possible to improve the accuracy of AD sampling), and the sampled data is sent to the FPGA for corresponding processing, thereby realizing the oscilloscope function; and the other signal after program-controlled gain enters the FPGA for corresponding processing through single-ended conversion, thereby realizing the high-frequency frequency counting function. [page]

Figure 2 shows the peripheral circuit of AD8260, where the AD8260 chip is powered by ±5 V dual power supply, and can also be powered by +3.3 V single power supply through jumper wires, that is, -5VS is grounded, R21 is welded, R22 is disconnected, and +5VS is connected to +3.3 V.

2 Design Circuit
The coupling selection and attenuation control circuit in the preamplifier circuit of the digital analyzer is shown in Figure 3, where the input signal enters from the CH1 channel, and after the overvoltage limiter of D1 and D2, the on and off of the K2 relay is controlled by GPI01 to select the DC and AC coupling modes of the oscilloscope respectively. Then it enters the 1x and 10x high-impedance attenuation (realized by GPI-02 controlling the K1 relay), where C1, C2, C3, R3, and R5 form a 10x high-impedance attenuation network. The attenuated signal can be directly sent to the AD8260 for digital programmable gain amplification. The 1x or 10x attenuated signal is input from the 17th and 18th pins of the AD8260, and after the fixed gain amplification of 6 dB by the internal front-end amplifier of the AD8260, -30 dB programmable attenuation, and 18 dB fixed gain amplification by the final amplifier, it will be output from the 7th and 8th pins. Therefore, the total gain range of the module is -6 to +24 dB. The digital programmable gain function block diagram of AD8260 is shown in Figure 4. Table 1 lists the gain adjustment truth table of AD8260.
After the signal is output from pins 7 and 8 of AD8260, one path is directly sent to three AD9433s in differential form for time-division high-speed AD sampling to realize the functions of 100 MHz oscilloscope and frequency meter below 20 MHz; the other path is converted into a single-ended signal through the resistor matching network composed of R19 and R20 and the T1 transformer and sent to FPGA to directly count the frequency of high-frequency signals. The FPGA of this design uses EP2C20 of ALTERA. The input high-precision clock signal is multiplied by the built-in phase-locked loop of FPGA. Its core clock is 600 MHz, and the frequency counting accuracy reaches 10-7. In PCB board making, because it is a high-frequency circuit design, in order to prevent the power supply of each module from interfering with the circuit, the power supply of each module must be isolated and filtered with inductors and capacitors. In addition, attention should be paid to the isolation of digital circuits and analog circuits.

3 Conclusion
The debugging in the actual system proves that the scheme introduced in this paper can effectively realize the program-controlled amplification of signals below 100 MHz, and the cost is low, which has certain practical significance.