LTC1966CMS8#TRPBF Linear Technology, LTC1966CMS8#TRPBF Datasheet - Page 14

• 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

LTC1966
applicaTions inForMaTion
A 1µF 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
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 pre-
vious section. If a ceramic capacitor is used, it may be
necessary 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 temperature.
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 < 0.1%
gain accuracy degradation, the parallel impedance of the
14
–0.2
–0.4
–0.6
–0.8
–2.0
–1.0
–1.2
–1.4
–1.6
–1.8
0
1
C = 100µF
C = 47µF
AVE
C = 22µF
Figure 8. Peak Error vs Input Frequency with One Cap Averaging
will generally be required.
C = 10µF
INPUT FREQUENCY (Hz)
10
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 metalized
polyester, will be a much better choice. Although more
expensive, and larger for a given value, the value stabil-
ity and low leakage make metal film capacitors a trouble
free choice.
With any type of capacitor, the self resonance of the capaci-
tor 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 por-
tion 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 = 4.7µF
20
C = 2.2µF
50
60
C = 1µF
1966 F08
1966fb
100