LTC1966HMS8-PBF View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC1966HMS8-PBF Datasheet PDF : 38 Pages
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LTC1966
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
LTC1966
IN1
RMS TO DC
IN2 CONVERSION
CHARGE
PUMP
170kΩ
OUTPUT
IINJECT DC
CAVE
1966 F21
Figure 21. Behavioral Block Diagram of LTC1966
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, VOOS that
is added, and should be subtracted from the RMS results,
either digitally, or by an analog means.
20
15
10
5mV MIN
5
ASYMPTOTES
SHIFTED +2.5mV
IDEAL
0
0
5
10
15
20
VIN (mV AC)
1966 F22b
Figure 22b. AC Transfer Function with IINJECT = 30nA
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.
20
15
5mV MIN
10
5
IDEAL
ASYMPTOTES
SHIFTED +2.5mV
0
–20 –15 –10 –5 0
5 10 15 20
VIN (mV DC)
1966 F22a
Figure 22a. DC Transfer Function with IINJECT = 30nA
10MΩ
5V
5V
VDD
LTC1966
IN1
OUT
85kΩ
IN2
OUTRTN
VSS GND EN
–5V
750k
10MΩ
1µF
10MΩ
5V
+
LTL1T4194494
–
84.5k
–5V
100pF
1966 F23
VOUT
Figure 23. Monotonic AC Response with Offset
and Gain Corrected
1966fb
24
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