Design of analog signal conditioning circuit for handheld digital storage oscilloscope

Oscilloscopes are the most commonly used instruments in the field of time domain measurement. Due to the advantages of digital storage oscilloscopes in data processing, waveform data retention and storage, and energy consumption, they have replaced traditional analog oscilloscopes and become the mainstream product in the oscilloscope market. Among them, handheld digital storage oscilloscopes are widely welcomed for their small size, easy to carry, rich functions, and convenient for field and on-site testing. The front-end analog signal conditioning circuit of a handheld oscilloscope is related to the bandwidth and vertical measurement accuracy of the oscilloscope. It is a key circuit unit of the oscilloscope. Here, a design scheme for the front-end analog signal conditioning circuit of a handheld oscilloscope is proposed.

1 Overall structure of system hardware

The overall hardware structure of the handheld digital oscilloscope is shown in Figure 1. It mainly consists of front-end analog signal conditioning circuit, high-speed A/D conversion and storage circuit, time base circuit, trigger circuit, sampling control circuit, DSP, CPU and its peripheral circuits.

The working principle of the handheld digital storage oscilloscope: the analog signal input from the outside is adjusted by the front-end signal conditioning circuit and sent to the high-speed A/D converter to convert it into a digital signal. The high-speed storage circuit then caches the digital signal output by the A/D converter. The acquisition clock of the A/D conversion and the reading and writing of the high-speed storage circuit are controlled by the trigger circuit and the time base circuit. The CPU sends the waveform data to be displayed to the LCD display screen and writes the waveform data to be saved to the non-volatile memory. The digital signal processor DSP can perform various analysis and processing on the waveform data. Among them, the front-end signal conditioning circuit can make the peak-to-peak value and bias voltage of the conditioned signal meet the input requirements of the A/D converter. In order to ensure that the oscilloscope input signal is not affected by the internal circuit of the oscilloscope and to prevent the oscilloscope from being damaged by high-voltage signals, the front-end signal conditioning circuit of the oscilloscope should also provide high input impedance, low output impedance, high-frequency compensation circuit and fault protection circuit. In addition, the handheld oscilloscope is powered by a battery, and low-power devices should be used in circuit design.

The analog signal conditioning circuit of the oscilloscope has the following characteristics: 1) The vertical sensitivity range is 5 mV/div to 5 V/div (1-2-5 steps, programmable); 2) The maximum input voltage is 350 Vp-p; 3) The input impedance is 1 MΩ/125pF; 4) The bandwidth is 10 MHz, and the bandwidth can be increased to 100 MHz by changing the high-frequency compensation capacitor; 5) It has fault protection function; 6) Low power consumption and power saving.

Figure 2 is a block diagram of the analog signal conditioning circuit of a handheld digital oscilloscope. The circuit is mainly composed of a passive attenuation network, a fault protection circuit, a broadband programmable amplifier, and an A/D conversion driver amplifier.

Since the measurement range of the oscilloscope is limited and an A/D converter is used, the input signal voltage requirement of the A/D converter is 1 Vp-p; its reference level is generally 1.25 V. Therefore, before the signal is sampled by the A/D converter, it must be attenuated or amplified. Here, the passive attenuation network provides four attenuation ratios: ÷1, ÷10, ÷100, and ÷1000. The oscilloscope main CPU controls the electronic switch to select according to different vertical sensitivity gears, and adjusts the signal to a suitable range suitable for circuit processing. The fault protection circuit provides an overvoltage protection function to prevent damage to components when the input voltage is too large. The main amplifier provides 2×, 4×, and 8× magnifications. The oscilloscope main CPU also controls the electronic switch to select according to different sensitivity gears. The conditioned signal is sent to the A/D conversion drive amplifier to generate a suitable level required for the input end of the A/D converter.

2 Oscilloscope analog signal conditioning circuit design

Figure 3 shows the design circuit of the analog signal conditioning part of the handheld digital oscilloscope.

2.1 Passive attenuation network

The passive attenuation network is composed of R1~R7, providing an input impedance of 1 MΩ. Switch SW1 is an AC/DC signal coupling switch. Capacitors C2~C6 are high-frequency compensation capacitors. By adjusting C2 and C3, high-frequency signals can be optimally compensated. Different vertical sensitivity gears of the oscilloscope select different attenuation ratios. The large gear (5~20V/div) selects ÷1000, and the medium gear (500mV/div~2V/div) selects ÷1000. div) selects ÷100, small gear (200～50mV／div) selects ÷10, minimum gear (20～5mV／div) selects ÷1, and the input signal attenuation multiple is set by the multiplexer MAX4534 and relay RE1. MAX4534 is a four-to-one multiplexer with fault protection function. Its truth table is shown in Table 1. The two control terminals A0 and A1 are controlled by the oscilloscope main CPU. The CPU sets the control words of the A0 and A1 ports according to different vertical sensitivity gears and selects one signal output. ATT0 and ATT1 are CPU control signals. When 50 mV／div is selected, the CPU writes 00 to the A0 and A1 ports, and the MAX4534 selects the second signal (pin 5) to output to the COM terminal (pin 7). At this time, the relationship between the voltage Uin input to the oscilloscope input terminal and the output voltage U0 of the MAX4534 is:

When the 5mV/div gear is selected, the CPU controls relay RELAY1 to close, so that the signal at the input end of the oscilloscope is directly sent to the main amplifier through resistor R8.

2.2 Fault protection circuit

In order to prevent excessive input voltage or other faults from damaging too many devices, the analog signal conditioning circuit of the handheld oscilloscope should have a fault protection circuit. The maximum allowable input voltage of MAX4534 is 40 V, and the power supply voltage is ±5V. Therefore, the maximum input voltage of its input terminal should be 35V. The signal sent to MAX4534 has been attenuated by 10 times at the minimum. Considering the impact of excessive voltage on passive devices in the attenuation network and PCB board wiring, the maximum voltage allowed to be input at the input terminal of the oscilloscope should be 350 Vrms. MAX4534 has a fault protection function. When the input voltage is too high, MAX4534 will work in power-down mode to prevent breakdown by excessive voltage. In Figure 3, the role of diodes VD1 and VD2 is to prevent excessive input voltage from damaging subsequent devices. When there is an excessive voltage input, the voltage between VD1 and VD2 will be clamped between ±Vcc to prevent high voltage signals from passing and prevent damage to subsequent devices.

2.3 Main amplifier and A/D conversion driver amplifier

The main amplifier circuit is composed of the broadband operational amplifier EL5160, the four-to-one multiplexer MAX4518 and the peripheral circuit. In order to reduce the signal distortion, ensure the bandwidth and take into account the low power consumption requirements of the handheld oscilloscope, the main amplifier should have high slew rate, low power consumption and amplitude change without affecting the bandwidth. Therefore, the current feedback broadband operational amplifier EL5160 is selected here.

The main amplifier provides 2x, 4x or 8x amplification for the signal sent by the attenuation network according to different vertical sensitivity levels. In this solution, the handheld oscilloscope provides 8x amplification when the vertical sensitivity level is 5 (5mV/d-ivr, 50mV/div, etc.), 4x amplification when the vertical sensitivity level is 1 (10mV/div, 1000mV/div, etc.), and 2x amplification when the vertical sensitivity level is 2 (20mV/div, 200mV/div, etc.). The amplification factor is determined by the feedback resistors R11~R13. The feedback resistors are selected by the 4-to-1 multiplexer MAX4518 in the circuit. The logic functions of MAX4518 and MAX4534 are exactly the same (see Table 1 for the truth table), except that the signal output is changed to pin 6, and it does not have a fault protection function. The CPU sets the control word of the MAX4518 A0 and A1 ports according to different vertical sensitivity levels to select a signal output. ATT2 and ATT3 in Figure 3 are CPU control signals. When the 100mV/div range is selected, the CPU writes 01 to the A0 and A1 ports. At this time, the MAX4534 selects the second signal (pin 5) to be output to the COM terminal (pin 6) through the resistor R12 and grounded. At this time, the relationship between the input voltage Uin1 of the main amplifier input terminal and the output voltage U01 is:

Similarly, when the control word of the A0 and A1 ports is 00 or 10, the MAX4534 selects the first signal (pin 5) or the third signal (pin 11) to be output to the COM terminal (pin 6) through resistor R11 or R13 and then grounded. At this time, the gain of the main amplifier is 8U0 or 2U0 respectively.

After the signal is amplified by the main amplifier, it still cannot meet the requirements of the A/D converter input end. Usually, the input signal voltage value is required to be about 1Vp-p. Therefore, the differential input proportional circuit composed of the broadband operational amplifier MAX4012 and its peripheral circuits is placed after the main amplifier. The input signal of the A/D converter requires that the input signal should have characteristics such as low offset voltage and low noise. Therefore, the voltage feedback operational amplifier MAX4012 is selected here. The signal input to its in-phase input end comes from the reference voltage signal (usually 1.25 V) output by the subsequent A/D converter itself, and its inverting input end is connected to the output end of the main amplifier through resistor R14. The signal after the A/D conversion drive amplifier can meet the requirements of the input end of the A/D converter. After the A/D conversion drive amplifier, the analog signal is sent to the input end of the A/D converter and converted into a digital signal.

3 Conclusion

The handheld oscilloscope prototype using this circuit design has a vertical measurement error of less than 1% in each gear during the test. Since this system design uses a high-speed, low-power operational amplifier, the bandwidth of the handheld oscilloscope can reach 100 MHz as long as the appropriate coupling capacitors and reasonable PCB board design are selected.