# What are the maximum voltage limits for TDS oscilloscopes+ and how do the limits change over the frequency range?

### 問題：

What are the maximum voltage limits for TDS oscilloscopes+ and how do the limits change over the frequency range?

### 答案：

The maximum input voltage listed in the performance verification procedure manuals for the 1 M Ohms input impedance is: ±400 V (DC + peak AC); derate at 20 dB per decade above 1 MHz.

The 50 Ohms input impedance limit is: 5 VRMS, with peaks less than or equal to ±30 V.

The limits for 1 M Ohms impedance and 50 Ohms impedance converge at 80 MHz. Above that frequency the derating curve continues down at the 20 dB per decade rate for both impedance settings.

Keep in mind that you will usually use a passive probe on the 1 M Ohms input, and that the probe voltage limits will apply. On a TDS754D with a P6139A, the voltage limit is 50 V above 20 MHz out to the probe bandwidth. The recommended active probes also have voltage limits, most less than 50 V.

These voltage limits are imposed by power dissipation limits in the input circuits. The 50 Ohms input impedance is a 1/2 W resistor. This power rating limits the voltage rating to 5 V RMS. On the 1 M Ohms input, as frequencies increase, the capacitive input impedance component decreases and input power is increasingly dissipated in the input capacitance. This heats the whole input attenuator hybrid. When using a probe, the power dissipation in the voltage divider resistors and capacitors in the probe, as well as in the oscilloscope input, limit the voltage. A 10X probe will usually have a 50 V limit at high frequencies because of power dissipated in the divider resistor and capacitor.