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PDF LTC1968 Data sheet ( Hoja de datos )
| Número de pieza | LTC1968 | |
| Descripción | RMS-to-DC Converter | |
| Fabricantes | Linear | |
| Logotipo |  |
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LTC1968
Precision Wide Bandwidth,
RMS-to-DC Converter
FEATURES
■ High Linearity:
0.02% Linearity Allows Simple System Calibration
■ Wide Input Bandwidth:
Bandwidth to 1% Additional Gain Error: 500kHz
Bandwidth to 0.1% Additional Gain Error: 150kHz
3dB Bandwidth Independent of Input Voltage
Amplitude
■ No-Hassle Simplicity:
True RMS-DC Conversion with Only One External
Capacitor
Delta Sigma Conversion Technology
■ Ultralow Shutdown Current:
0.1µA
■ Flexible Inputs:
Differential or Single Ended
Rail-to-Rail Common Mode Voltage Range
Up to 1VPEAK Differential Voltage
■ Flexible Output:
Rail-to-Rail Output
Separate Output Reference Pin Allows Level Shifting
■ Small Size:
Space Saving 8-Pin MSOP Package
U
APPLICATIO S
■ True RMS Digital Multimeters and Panel Meters
■ True RMS AC + DC Measurements
DESCRIPTIO
The LTC®1968 is a true RMS-to-DC converter that uses an
innovative delta-sigma computational technique. The ben-
efits of the LTC1968 proprietary architecture, when com-
pared to conventional log-antilog RMS-to-DC converters,
are higher linearity and accuracy, bandwidth independent
of amplitude and improved temperature behavior.
The LTC1968 operates with single-ended or differential in-
put signals and accurately supports crest factors up to 4.
Common mode input range is rail-to-rail. Differential in-
put range is 1VPEAK, and offers unprecedented linearity. The
LTC1968 allows hassle-free system calibration at any in-
put voltage.
The LTC1968 has a rail-to-rail output with a separate out-
put reference pin providing flexible level shifting; it oper-
ates on a single power supply from 4.5V to 5.5V. A low power
shutdown mode reduces supply current to 0.1µA.
The LTC1968 is packaged in the space-saving MSOP pack-
age, which is ideal for portable applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Protected under U.S. Patent Numbers 6,359,576, 6,362,677 and 6,516,291
TYPICAL APPLICATIO
Single Supply RMS-to-DC Converter
4.5V TO 5.5V
DIFFERENTIAL
INPUT
0.1µF
OPT. AC
COUPLING
V+
IN1 OUTPUT
LTC1968
IN2 OUT RTN
EN GND
1968 TA01
CAVE
10µF
+
–
VOUT
Linearity Performance
0.2
LTC1968, ∆Σ
0
–0.2
–0.4
–0.6 CONVENTIONAL
LOG/ANTILOG
–0.8
60Hz SINEWAVE
–1.0
0 100 200
300
VIN (mV ACRMS)
400 500
1968 TA01b
1968f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LTC1968
Performance vs Crest Factor
201.0
200.8
200.6
200mVRMS SCR WAVEFORMS
CAVE = 10µF
O.1%/DIV
200.4
20Hz
200.2
200.0
199.8
199.6
1kHz
10kHz
60Hz
199.4
199.2
199.0
1
23 4
CREST FACTOR
5
1968 G05
DC Linearity
0.10
CAVE = 10µF
0.08 VIN2 = MIDSUPPLY
0.06
0.04
0.02
0
–0.02
–0.04
–0.06 EFFECT OF OFFSETS
–0.08 MAY BE POSITIVE OR
NEGATIVE AT VIN = 0V
–0.10
–500 –300 –100
100
VIN1 (mV)
300 500
1968 G08
Power Supply and ENABLE Pin
Current vs ENABLE Voltage
3.0
300
2.5 200
2.0
IS 100
1.5
IEN 0
1.0
–100
0.5
0 –200
–0.5 –300
–1.0
0
1 23 45
ENABLE PIN VOLTAGE (V)
–400
6
1968 G11
Performance vs Large Crest Factor
220
20Hz
210
10kHz
200
190 40kHz
60Hz
1kHz
180
170
160
150
140 200mVRMS SCR WAVEFORMS
130 CAVE = 10µF
5%/DIV
120
12 3 4 5 6 7 8
CREST FACTOR
1968 G06
Supply Current vs Supply Voltage
3.0
2.5
2.0
1.5
1.0
0.5
0
012 34 56
SUPPLY VOLTAGE (V)
1968 G09
AC Linearity
0.20
SINEWAVES
0.15 CAVE = 10µF
VIN2 = MIDSUPPLY
0.10
0.05
0
–0.05
60Hz
40kHz
–0.10
–0.15
–0.20
0
100 200 300 400 500
VIN1 (mV ACRMS)
1968 G07
Supply Current vs Temperature
2.44
2.42
2.40
2.38
2.36
2.34
2.32
2.30
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
1968 G10
Input Signal Bandwidth
vs RMS Value
1000
–3dB
1% ERROR
100
1% ERROR
10
1k
10k 100k 1M 10M 100M
INPUT SIGNAL FREQUENCY (Hz)
1968 G12
Input Signal Bandwidth
202
200
198
196
194
192
190
188
186 1%/DIV
184 CAVE = 10µF
182 VIN = 200mVRMS
100 1k 10k 100k 1M 10M
INPUT SIGNAL FREQUENCY (Hz)
1968 G13
1968f
5
5 Page 
LTC1968
APPLICATIO S I FOR ATIO
0
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
1
C = 220µF
C = 100µF
C = 47µF
C = 22µF
C = 10µF
C = 4.7µF
C = 2.2µF
10 100
INPUT FREQUENCY (Hz)
Figure 8. Peak Error vs Input Frequency with One Cap Averaging
C =1µF
1000
1968 F08
because of the computation of the square of the input. The
typical values shown, 5% peak ripple with 0.05% DC error,
occur with CAVE = 10µF and fINPUT = 6Hz.
If the application calls for the output of the LTC1968 to feed
a sampling or Nyquist A/D converter (or other circuitry
that will not average out this double frequency ripple) a
larger averaging capacitor can be used. This trade-off is
depicted in Figure 8. The peak ripple error can also be
reduced by additional lowpass filtering after the LTC1968,
but the simplest solution is to use a larger averaging
capacitor.
A 10µF 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 CAVE will generally be
required. See “Crest Factor and AC + DC Waveforms.”
Capacitor Type Selection
The LTC1968 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
LTC1968 will create a constant offset of the output voltage.
If the leak is Ohmic, the resistor divider formed with the
LTC1968 output impedance will cause a gain error. For
< 0.1% gain accuracy degradation, the parallel impedance
of the capacitor leakage will need to be >1000 times the
LTC1968 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
LTC1968. 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 LTC1968 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.
1968f
11
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LTC1968.PDF ] | |
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