Datasheet

LTC1966
24
1966fb
applicaTions inForMaTion
To do this, inject current into the output. As shown in
Figure 21, the charge pump output impedance is 170kΩ,
with the computational feedback cutting the closed loop
output impedance to the 85kΩ specification. By injecting
30nA of current into this 170Ω, with zero input, a 5mV offset
Figure 23 shows an analog implementation of this with
the offset and gain errors corrected; only the slight, but
necessary, degradation in nonlinearity remains. The cir-
cuit works by creating approximately 300mV of bias at
the junction of the 10MΩ resistors when the LTC1966’s
input/output are zero. The 10MΩ resistor to the LTC1966
output therefore feeds in 30nA. The loading of this resis-
tor causes a slight reduction in gain which is corrected,
as is the nominal 2.5mV offset, by the LT1494 op amp.
is created at the output feedback point, which is sufficient
to overcome the 5mV minimum signal level. With large
enough input signals, the computational feedback cuts
the output impedance to 85kΩ so the transfer function
asymptotes will have an output offset of 2.5mV, as shown
in Figure 22. This is the additional, predictable, V
OOS
that
is added, and should be subtracted from the RMS results,
either digitally, or by an analog means.
Figure 21. Behavioral Block Diagram of LTC1966
Figure 22a. DC Transfer Function with I
INJECT
= 30nA
Figure 22b. AC Transfer Function with I
INJECT
= 30nA
Figure 23. Monotonic AC Response with Offset
and Gain Corrected
1µF
1966 F23
5V
–5V
5V
85kΩ
10MΩ
10MΩ
LT1494
750k
V
OUT
84.5k
100pF
OUT
LTC1966
V
DD
V
SS
ENGND
OUTRTN
IN1
IN2
–
+
10MΩ
LT1494
5V
–5V
RMS TO DC
CONVERSION
C
AVE
I
INJECT
DC
170kΩ
CHARGE
PUMP
LTC1966
OUTPUT
IN1
IN2
1966 F21
V
IN
(mV DC)
–20
0
V
OUT
(mV DC)
10
20
15
–15
–10
20
1966 F22a
5
0 5
–5
10
15
5mV MIN
IDEAL
ASYMPTOTES
SHIFTED +2.5mV
V
IN
(mV AC)
0
0
V
OUT
(mV DC)
10
20
15
5
20
1966 F22b
5
10 15
5mV MIN
IDEAL
ASYMPTOTES
SHIFTED +2.5mV