Specifications

2.3.2 Auto-balancing bridge section
The auto-balancing bridge section balances the range resistor current with the DUT current while
maintaining a zero potential at the Low terminal. Figure 2-5 (a) shows a simplified circuit model
that expresses the operation of the auto-balancing bridge. If the range resistor current is not bal-
anced with the DUT current, an unbalance current that equals Ix – Ir flows into the null detector at
the Lp terminal. The unbalance current vector represents how much the magnitude and phase angle
of the range resistor current differ from the DUT current. The null detector detects the unbalance
current and controls both the magnitude and phase angle of the OSC2 output so that the detected
current goes to zero.
Low frequency instruments, below 100 kHz, employ a simple operational amplifier to configure the
null detector and the equivalent of OSC2 as shown in Figure 2-5 (b). This circuit configuration
cannot be used at frequencies higher than 100 kHz because of the performance limits of the opera-
tional amplifier. The instruments that cover frequencies above 100 kHz have an auto balancing
bridge circuit consisting of a null detector, 0°/90° phase detectors, and a vector modulator as shown
in Figure 2-5 (c). When an unbalance current is detected with the null detector, the phase detectors
in the next stage separate the current into 0° and 90° vector components. The phase detector output
signals go through loop filters (integrators) and are applied to the vector modulator to drive the
0°/90° component signals. The 0°/90° component signals are compounded and the resultant
signal is fed back through range resistor (Rr) to cancel the current flowing through the DUT. Even if
the balancing control loop has phase errors, the unbalance current component, due to the phase
errors, is also detected and fed back to cancel the error in the range resistor current. Consequently,
the unbalance current converges to exactly zero, ensuring Ix = Ir over a broad frequency range up to
110 MHz.
If the unbalance current flowing into the null detector exceeds a certain threshold level, the unbal-
ance detector after the null detector annunciates the unbalance state to the digital control section of
the instrument. As a result, an error message such as “OVERLOAD” or “BRIDGE UNBALANCED” is
displayed.
Figure 2-5. Auto-balancing bridge section block diagram
DUT
I
x
(a) Operation image of the auto-balancing bridge
H
c
L
c
L
p
I
r
R
r
V
X
V
r
Null
detector
(b) Auto-balancing bridge for frequency below 100 kHz
DUT
R
r
H
c
L
c
L
p
H
p
V
X
V
r
OSC
(c) Auto-balancing bridge for frequency above 100 kHz
DUT
R
r
H
c
L
c
L
p
H
p
V
X
V
r
OSC
–90°
Null detector
90 °0°
Phase
detector
Vector modulator
OSC1 OSC2
2-7