LTC1966CMS8#TRPBF Linear Technology, LTC1966CMS8#TRPBF Datasheet - Page 24

• Manufacturer Part Number: LTC1966CMS8#TRPBF
• Description: IC PREC RMS/DC CONV MCRPWR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1966CMS8#TRPBF

• Current - Supply: 155µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

applicaTions inForMaTion
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, V
is added, and should be subtracted from the RMS results,
either digitally, or by an analog means.
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
24
IN1
IN2
LTC1966
Figure 22a. DC Transfer Function with I
CONVERSION
Figure 21. Behavioral Block Diagram of LTC1966
RMS TO DC
20
15
10
5
0
–20
–15
IDEAL
–10
–5
V
IN
CHARGE
PUMP
(mV DC)
5mV MIN
0
170k
5
SHIFTED +2.5mV
ASYMPTOTES
10
OUTPUT
INJECT
15
1966 F21
1966 F22a
20
= 30nA
OOS
I
INJECT
C
AVE
that
DC
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.
IN1
IN2
Figure 22b. AC Transfer Function with I
V
–5V
SS
Figure 23. Monotonic AC Response with Offset
and Gain Corrected
LTC1966
20
15
10
V
GND
5V
5
0
DD
0
OUTRTN
5mV MIN
EN
OUT
5V
10M
85k
5
IDEAL
V
10M
IN
1µF
SHIFTED +2.5mV
(mV AC)
10
ASYMPTOTES
10M
100pF
750k
84.5k
15
–
+
LT1494
LT1494
INJECT
–5V
5V
1966 F22b
20
= 30nA
1966 F23
1966fb
V
OUT