LTC1966CMS8 Linear Technology, LTC1966CMS8 Datasheet - Page 14

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

Specifications of LTC1966CMS8

• 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: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC1966
A 1 F capacitor is a good choice for many applications.
The peak error at 50Hz/60Hz will be <1% and the DC error
will be <0.1% with frequencies of 10Hz or more.
Note that both Figure 6 and Figure 8 assume AC-coupled
waveforms with a crest factor less than 2, such as sine
waves or triangle waves. For higher crest factors and/or
AC + DC waveforms, a larger C
required. See “Crest Factor and AC + DC Waveforms.”
Capacitor Type Selection
The LTC1966 can operate with many types of capacitors.
The various types offer a wide array of sizes, tolerances,
parasitics, package styles and costs.
Ceramic chip capacitors offer low cost and small size, but
are not recommended for critical applications. The value
stability over voltage and temperature is poor with many
types of ceramic dielectrics. This will not cause an RMS-
to-DC accuracy problem except at low frequencies, where
it can aggravate the effects discussed in the previous
section. If a ceramic capacitor is used, it may be neces-
sary to use a much higher nominal value in order to
assure the low frequency accuracy desired.
Another parasitic of ceramic capacitors is leakage, which
is again dependent on voltage and particularly tempera-
ture. If the leakage is a constant current leak, the I • R drop
of the leak multiplied by the output impedance of the
LTC1966 will create a constant offset of the output voltage.
If the leak is Ohmic, the resistor divider formed with the
LTC1966 output impedance will cause a gain error. For
14
–0.2
–0.4
–0.6
–2.0
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
0
1
C = 100 F
C = 47 F
U
U
C = 22 F
Figure 8. Peak Error vs Input Frequency with One Cap Averaging
AVE
W
will generally be
U
C = 10 F
INPUT FREQUENCY (Hz)
10
< 0.1% gain accuracy degradation, the parallel impedance
of the capacitor leakage will need to be >1000 times the
LTC1966 output impedance. Accuracy at this level can be
hard to achieve with a ceramic capacitor, particularly with
a large value of capacitance and at high temperature.
For critical applications, a film capacitor, such as metal-
ized polyester, will be a much better choice. Although
more expensive, and larger for a given value, the value
stability and low leakage make metal-film capacitors a
trouble-free choice.
With any type of capacitor, the self-resonance of the
capacitor can be an issue with the switched capacitor
LTC1966. If the self-resonant frequency of the averaging
capacitor is 1MHz or less, a second smaller capacitor
should be added in parallel to reduce the impedance seen
by the LTC1966 output stage at high frequencies. A
capacitor 100 times smaller than the averaging capacitor
will typically be small enough to be a low cost ceramic with
a high quality dielectric such as X7R or NPO/COG.
Input Connections
The LTC1966 input is differential and DC coupled. The
LTC1966 responds to the RMS value of the differential
voltage between Pin 2 and Pin 3, including the DC portion
of that difference. However, there is no DC-coupled path
from the inputs to ground. Therefore, at least one of the two
inputs must be connected with a DC-return path to ground.
Both inputs must be connected to something. If either
input is left floating, a zero volt output will result.
C = 4.7 F
20
C = 2.2 F
50
60
C = 1 F
sn1966 1966fas
1966 F08
100