Tektronix P7313SMA, Technical Reference

Page: 17 / 62

Operating Basics
The probe block diagram shows that the input termination network is followed by an attenuator and V
CM
compensation circuit.
The attenuator is used to increase the effective input dynamic range of the probe differential ampli fi er.
The P7313SMA probe has two attenuation settings, 2.5X and 12.5X, that allow dynamic range to be traded off against signal
noise. The 12.5X attenuator setting has the largest dynamic range; the 2.5X attenuator setting has the lowest noise.
The V
CM
compensation circuit automatically minimizes the DC common-mode voltage at the probe differential ampli fi er inputs
even with varying termination voltage and input signal DC common-mode voltage. This maximizes the differential mode
signal input dynamic range. The V
CM
compensation circuit allows the DC common-mode input voltage range to be the same
for both attenuator settings. (See Figure 4 on page 7.)
Internal Probe Ampli fi er
The P7313SMA differential probe is designed to measure high frequency, low-voltage circuits. Before connecting the probe to
your circuit, take into account the limits for maximum input voltage, the common-mode signal range, and the differential-mode
signal range. For speci
fi c limits of these parameters, see the Speci fi cations section. (See page 25, Speci fi cations .)
Maximum Input Voltage.
The maximum input voltage is the maximum voltage to ground that the inputs can withstand
without damaging the probe input circuitry.
CAUTION. To avoid damaging the inputs of the P7313SMA differential probe, do not apply more than ±5 V (DC + peak AC)
between each input and ground. In addition, to avoid probe damage, the maximum termination resistor power must not
be exceeded.
Maximum Termination Resistor Power.
The internal termination resistors can safely dissipate 0.2 W of power
continuously, which is the case for normal probe operation without termination driver current overload. However, the probe
will be damaged if you apply more than 0.5 W of power through the termination resistors for more than 5 minutes.
If you suspect your measurement application will approach these limits, use the formulas that follow to calculate the power
dissipated by the termination resistors.
The power calculation formulas are based on the simpli fi ed model, which represents the signal at the probe inputs. (See
Figure 3 on page 6.) If a signal source with 50 Ω source impedances is used, the signal levels used should match the
zero-ohm source impedance model shown in the fi gure.
The signal source model de fi ned for these equations is as follows:
P7313SMA Technical Reference 5