Agilent Technologies 664xA, 665xA, 667xA, 669xA, 668xA manual Connecting the Sense Leads

Connecting the Sense Leads

You must connect the positive side of the load to the +S analog connector pin and the negative side of the load to the -S analog connector pin (see Figure 4-1). Connect the sense leads carefully so that they do not become open-circuited. If sense leads are left open during operation, the supply will regulate at the output terminals instead of at the load. Remember to bundle or tie wrap the load leads to minimize inductance and reduce noise pickup.

CV Regulation

The voltage load regulation specification in Table 1-3a applies at the output terminals of the power supply. When remote sensing, this specification must be compensated. Add an increment to the voltage load regulation specification as specified by “∆mV” in the equation given under Load regulation in Table 1-3b.

OVP Considerations

The OVP circuit senses the voltage near the output terminals and not at the sense terminals. Depending on the voltage drop between the output terminals and the load, the voltage sensed by the OVP circuit can be significantly higher than actually being regulated at the load. You must program the OVP trip high enough to compensate for the expected higher voltage at the output terminals.

Output Rating

The rated output voltage and current specification in Table 1-3a applies at the output terminals of the power supply. With remote sensing, any voltage dropped in the load leads causes the supply to increase the voltage at the output terminals so it can maintain the proper voltage at the load. When you attempt to operate at the full-rated output at the load, this forces the supply voltage at the output terminals to exceed the supply's rated output. This will not damage the supply, but may trip the OVP (overvoltage protection) circuit, which senses the voltage at the output bus bars. When operated beyond its rated output, the supply's performance specifications are not guaranteed, although typical performance may be good. If the excessive demand on the supply forces it to lose regulation, the Unr annunciator will indicate that the output is unregulated.

Output Noise

Any noise picked up on the sense leads also appears at the output of the power supply and may adversely affect the load voltage regulation. Be sure to twist the sense leads to minimize external noise pickup and route them parallel and close to the load leads. In noisy environments, it may be necessary to shield the sense leads. Ground the shield only at the power supply. Do not use the shield as one of the sense conductors.

Note The signal ground binding post on the rear panel is a convenient place to ground the sense shield.

Stability

Using remote sensing under unusual combinations of load-lead lengths and large load capacitances may cause your application to form a low-pass filter that becomes part of the voltage feedback loop. The extra phase shift created by this filter can degrade the supply's stability and result in poor transient response. In severe cases, this may cause output oscillations. To minimize this possibility, keep the load leads as short as possible and tie wrap them together.

In most cases, following the above guidelines will prevent problems associated with load lead inductance. However, if a large bypass capacitor is required at the load and load-lead length cannot be reduced, then a sense-lead bypass network may be needed to ensure stability (see Figure 4-4b). The voltage rating of the 33 ∝F capacitors should be about 50% greater than the anticipated load-lead drop. Addition of the 20-Ω resistors will cause a slight voltage rise at the remote sensing points. For utmost voltage programming accuracy, the supply should be recalibrated with the DVM at the remote sensing points (see “Appendix A - Calibration”).

Note	If you need help in solving a stability problem with any Series 667xA power supply contact an Agilent
	Service Engineer through your local Agilent Sales and Support Offices.

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