FEATURES
s
LTC1966
Precision Micropower,
∆Σ
RMS-to-DC Converter
DESCRIPTIO
The LTC
®
1966 is a true RMS-to-DC converter that utilizes
an innovative patented
∆Σ
computational technique. The
internal delta-sigma circuitry of the LTC1966 makes it sim-
pler to use, more accurate, lower power and dramatically
more flexible than conventional log-antilog RMS-to-DC
converters.
The LTC1966 accepts single ended or differential input
signals (for EMI/RFI rejection) and supports crest factors
up to 4. Common mode input range is rail-to-rail. Differ-
ential input range is 1V
PEAK
, and offers unprecedented lin-
earity. Unlike previously available RMS-to-DC converters,
the superior linearity of the LTC1966 allows hassle-free
system calibration at any input voltage.
The LTC1966 also has a rail-to-rail output with a separate
output reference pin providing flexible level shifting. The
LTC1966 operates on a single power supply from 2.7V to
5.5V or dual supplies up to
±5.5V.
A low power shutdown
mode reduces supply current to 0.5µA.
The LTC1966 is insensitive to PC board soldering and
stresses, as well as operating temperature. The LTC1966
is packaged in the space-saving MSOP package which is
ideal for portable applications.
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APPLICATIO S
s
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True RMS Digital Multimeters and Panel Meters
True RMS AC + DC Measurements
, LTC and LT are registered trademarks of Linear Technology Corporation.
Protected under U.S. Patent Numbers 6,359,576 and 6,362,677
TYPICAL APPLICATIO
Quantum Leap in Linearity Performance
LINEARITY ERROR (V
OUT
mV DC – V
IN
mV AC
RMS
)
0.2
0
LTC1966,
∆Σ
Single Supply RMS-to-DC Converter
2.7V TO 5.5V
V
DD
IN1
DIFFERENTIAL
INPUT
0.1µF
OPT. AC
COUPLING
IN2
EN
OUTPUT
LTC1966
OUT RTN
V
SS
GND
C
AVE
1µF
1966 TA01
–0.2
–0.4
–0.6
–0.8
–1.0
0
60Hz SINEWAVES
50 100 150 200 250 300 350 400 450 500
V
IN
(mV AC
RMS
)
1966 TA01b
CONVENTIONAL
LOG/ANTILOG
+
V
OUT
–
U
No-Hassle Simplicity:
True RMS-DC Conversion with Only One External
Capacitor
Delta Sigma Conversion Technology
High Accuracy:
0.1% Gain Accuracy from 50Hz to 1kHz
0.25% Total Error from 50Hz to 1kHz
High Linearity:
0.02% Linearity Allows Simple System Calibration
Low Supply Current:
155µA Typ, 170µA Max
Ultralow Shutdown Current:
0.1µA
Constant Bandwidth:
Independent of Input Voltage
800kHz –3dB, 6kHz
±1%
Flexible Supplies:
2.7V to 5.5V Single Supply
Up to
±5.5V
Dual Supply
Flexible Inputs:
Differential or Single Ended
Rail-to-Rail Common Mode Voltage Range
Up to 1V
PEAK
Differential Voltage
Flexible Output:
Rail-to-Rail Output
Separate Output Reference Pin Allows Level Shifting
Small Size:
Space Saving 8-Pin MSOP Package
U
U
sn1966 1966fas
1
LTC1966
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
GND
IN1
IN2
V
SS
1
2
3
4
8
7
6
5
ENABLE
V
DD
OUT RTN
V
OUT
Supply Voltage
V
DD
to GND .............................................. – 0.3 to 7V
V
DD
to V
SS
............................................ – 0.3V to 12V
V
SS
to GND ............................................. – 7V to 0.3V
Input Currents (Note 2) .....................................
±10mA
Output Current (Note 3) .....................................
±10mA
ENABLE Voltage ..................... V
SS
– 0.3V to V
SS
+ 12V
OUT RTN Voltage .............................. V
SS
– 0.3V to V
DD
Operating Temperature Range (Note 4)
LTC1966C/LTC1966I ......................... – 40°C to 85°C
Specified Temperature Range (Note 5)
LTC1966C/LTC1966I ......................... – 40°C to 85°C
Maximum Junction Temperature ......................... 150°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LTC1966CMS8
LTC1966IMS8
MS8 PART MARKING
LTTG
LTTH
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 150°C,
θ
JA
= 220°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
G
ERR
V
OOS
LIN
ERR
PSRR
V
IOS
PARAMETER
Conversion Gain Error
Output Offset Voltage
Linearity Error
Power Supply Rejection
Input Offset Voltage
Conversion Accuracy
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
DD
= 5V, V
SS
= – 5V, V
OUTRTN
= 0V, C
AVE
= 10µF, V
IN
= 200mV
RMS
,
V
ENABLE
= 0.5V unless otherwise noted.
CONDITIONS
50Hz to 1kHz Input (Notes 6, 7)
q
MIN
TYP
±0.1
0.1
MAX
±0.3
±0.4
0.2
0.4
0.15
0.15
0.20
0.8
1.0
2
30
V
DD
UNITS
%
%
mV
mV
%
%/V
%/V
mV
mV
mV
mV
V
MΩ
MΩ
(Notes 6, 7)
q
50mV to 350mV (Notes 7, 8)
(Note 9)
q
q
0.02
0.02
0.2
(Notes 6, 7, 10)
q
Accuracy vs Crest Factor (CF)
CF = 4
CF = 5
Input Characteristics
I
VR
Z
IN
CMRRI
V
IMAX
V
IMIN
PSRRI
Input Voltage Range
Input Impedance
Input Common Mode Rejection
Maximum Input Swing
Minimum RMS Input
Power Supply Rejection
V
DD
Supply (Note 9)
V
SS
Supply (Note 9)
Average, Differential (Note 12)
Average, Common Mode (Note 12)
(Note 13)
Accuracy = 1% (Note 14)
q
q
q
q
q
q
60Hz Fundamental, 200mV
RMS
(Note 11)
60Hz Fundamental, 200mV
RMS
(Note 11)
q
q
–1
– 20
V
SS
8
100
7
1
1.05
200
5
250
120
600
300
sn1966 1966fas
2
U
µV/V
V
mV
µV/V
µV/V
W
U
U
W W
W
LTC1966
ELECTRICAL CHARACTERISTICS
SYMBOL
OVR
Z
OUT
CMRRO
V
OMAX
PSRRO
PARAMETER
Output Voltage Range
Output Impedance
Output Common Mode Rejection
Maximum Differential Output Swing
Power Supply Rejection
(Note 12)
(Note 13)
Accuracy = 2%, DC Input (Note 14)
q
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
DD
= 5V, V
SS
= – 5V, V
OUTRTN
= 0V, C
AVE
= 10µF, V
IN
= 200mV
RMS
,
V
ENABLE
= 0.5V unless otherwise noted.
CONDITIONS
q
q
q
MIN
V
SS
75
1.0
0.9
TYP
MAX
V
DD
UNITS
V
kΩ
µV/V
V
V
Output Characteristics
85
16
1.05
250
50
6
20
800
q
95
200
V
DD
Supply (Note 9)
V
SS
Supply (Note 9)
C
AVE
= 10µF
C
AVE
= 10µF
q
q
1000
500
µV/V
µV/V
kHz
kHz
kHz
Frequency Response
f
1P
f
10P
f
– 3dB
V
DD
V
SS
I
DD
I
SS
I
DDS
I
SSS
I
IH
I
IL
V
TH
1% Additional Error (Note 15)
10% Additional Error (Note 15)
±3dB
Frequency (Note 15)
Positive Supply Voltage
Negative Supply Voltage
Positive Supply Current
Negative Supply Current
Supply Currents
Supply Currents
ENABLE Pin Current High
ENABLE Pin Current Low
ENABLE Threshold Voltage
(Note 16)
IN1 = 20mV, IN2 = 0V
IN1 = 200mV, IN2 = 0V
IN1 = 20mV, IN2 = 0V
V
ENABLE
= 4.5V
V
ENABLE
= 4.5V
V
ENABLE
= 4.5V
V
ENABLE
= 0.5V
V
DD
= 5V, V
SS
= – 5V
V
DD
= 5V, V
SS
= GND
V
DD
= 2.7V, V
SS
= GND
2.7
– 5.5
155
158
12
0.5
–1
– 0.3
–2
– 0.1
– 0.05
–1
2.4
2.1
1.3
0.1
– 0.1
Power Supplies
5.5
0
170
20
10
V
V
µA
µA
µA
µA
µA
µA
µA
V
V
V
V
q
q
q
Shutdown Characteristics
q
q
q
q
V
HYS
ENABLE Threshold Hysteresis
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The inputs (IN1, IN2) are protected by shunt diodes to V
SS
and
V
DD
. If the inputs are driven beyond the rails, the current should be limited
to less than 10mA.
Note 3:
The LTC1966 output (V
OUT
) is high impedance and can be
overdriven, either sinking or sourcing current, to the limits stated.
Note 4:
The LTC1966C/LTC1966I are guaranteed functional over the
operating temperature range of – 40°C to 85°C.
Note 5:
The LTC1966C is guaranteed to meet specified performance from
0°C to 70°C. The LTC1966C is designed, characterized and expected to
meet specified performance from – 40°C to 85°C but is not tested nor QA
sampled at these temperatures. The LTC1966I is guaranteed to meet
specified performance from – 40°C to 85°C.
Note 6:
High speed automatic testing cannot be performed with
C
AVE
= 10µF. The LTC1966 is 100% tested with C
AVE
= 22nF. Correlation
tests have shown that the performance limits above can be guaranteed
with the additional testing being performed to guarantee proper operation
of all the internal circuitry.
Note 7:
High speed automatic testing cannot be performed with 60Hz
inputs. The LTC1966 is 100% tested with DC and 10kHz input signals.
Measurements with DC inputs from 50mV to 350mV are used to calculate
the four parameters: G
ERR
, V
OOS
, V
IOS
and linearity error. Correlation tests
have shown that the performance limits above can be guaranteed with the
additional testing being performed to guarantee proper operation of all
internal circuitry.
Note 8:
The LTC1966 is inherently very linear. Unlike older log/antilog
circuits, its behavior is the same with DC and AC inputs, and DC inputs are
used for high speed testing.
Note 9:
The power supply rejections of the LTC1966 are measured with
DC inputs from 50mV to 350mV. The change in accuracy from V
DD
= 2.7V
to V
DD
= 5.5V with V
SS
= 0V is divided by 2.8V. The change in accuracy
from V
SS
= 0V to V
SS
= –5.5V with V
DD
= 5.5V is divided by 5.5V.
Note 10:
Previous generation RMS-to-DC converters required nonlinear
input stages as well as a nonlinear core. Some parts specify a “DC reversal
error,” combining the effects of input nonlinearity and input offset voltage.
The LTC1966 behavior is simpler to characterize and the input offset
voltage is the only significant source of “DC reversal error.”
sn1966 1966fas
3
LTC1966
ELECTRICAL CHARACTERISTICS
Note 11:
High speed automatic testing cannot be performed with 60Hz
inputs. The LTC1966 is 100% tested with DC stimulus. Correlation tests
have shown that the performance limits above can be guaranteed with the
additional testing being performed to verify proper operation of all internal
circuitry.
Note 12:
The LTC1966 is a switched capacitor device and the input/output
impedance is an average impedance over many clock cycles. The input
impedance will not necessarily lead to an attenuation of the input signal
measured. Refer to the Applications Information section titled “Input
Impedance” for more information.
Note 13:
The common mode rejection ratios of the LTC1966 are measured
with DC inputs from 50mV to 350mV. The input CMRR is defined as the
change in V
IOS
measured between input levels of V
SS
to V
SS
+ 350mV and
input levels of V
DD
– 350mV to V
DD
divided by V
DD
– V
SS
– 350mV. The
output CMRR is defined as the change in V
OOS
measured with OUT RTN =
V
SS
and OUT RTN = V
DD
– 350mV divided by V
DD
– V
SS
– 350mV.
Note 14:
The LTC1966 input and output voltage swings are limited by
internal clipping. However, its
∆Σ
topology is relatively tolerant of
momentary internal clipping. The input clipping is tested with a crest
factor of 2, while the output clipping is tested with a DC input.
Note 15:
The LTC1966 exploits oversampling and noise shaping to reduce
the quantization noise of internal 1-bit analog-to-digital conversions. At
higher input frequencies, increasingly large portions of this noise are
aliased down to DC. Because the noise is shifted in frequency, it becomes
a low frequency rumble and is only filtered at the expense of increasingly
long settling times. The LTC1966 is inherently wideband, but the output
accuracy is degraded by this aliased noise. These specifications apply with
C
AVE
= 10µF and constitute a 3-sigma variation of the output rumble.
Note 16:
The LTC1966 can operate down to 2.7V single supply but cannot
operate at
±2.7V.
This additional constraint on V
SS
can be expressed
mathematically as – 3 • (V
DD
– 2.7V)
≤
V
SS
≤
Ground.
TYPICAL PERFOR A CE CHARACTERISTICS
Gain and Offset
vs Input Common Mode
0.5
0.4
0.3
V
DD
= 5V
V
SS
= GND
0.5
V
IOS
V
OOS
GAIN ERROR
0.4
0.3
0.5
GAIN ERROR (%)
GAIN ERROR (%)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.5
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
INPUT COMMON MODE (V)
1966 G02
Gain and Offset
vs Output Common Mode
0.5
0.4
0.3
V
DD
= 5V
V
SS
= GND
V
IOS
V
OOS
GAIN ERROR
0.5
0.4
0.3
0.5
GAIN ERROR (%)
GAIN ERROR (%)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.5
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
OUTPUT COMMON MODE (V)
1966 G05
4
U W
Gain and Offset
vs Input Common Mode
V
DD
= 5V
0.4 V
SS
= –5V
0.3
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
OFFSET VOLTAGE (mV)
OFFSET VOLTAGE (mV)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–5 –4 –3 –2 –1 0 1 2 3
INPUT COMMON MODE (V)
4
5
GAIN ERROR
V
OOS
V
IOS
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
1966 G03
Gain and Offset
vs Output Common Mode
V
DD
= 5V
0.4 V
SS
= –5V
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–5 –4 –3 –2 –1 0 1 2 3
OUTPUT COMMON MODE (V)
4
5
V
IOS
GAIN ERROR
V
OOS
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
1966 G06
sn1966 1966fas
LTC1966
TYPICAL PERFOR A CE CHARACTERISTICS
Gain and Offsets vs Temperature
0.5
V
DD
= 5V
0.4 V
SS
= GND
0.3
GAIN ERROR (%)
GAIN ERROR (%)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–50
–25
50
75
25
0
TEMPERATURE (°C)
Gain and Offset
vs Input Common Mode
0.5
0.4
0.3
V
DD
= 2.7V
V
SS
= GND
1.0
0.8
V
IOS
GAIN ERROR
0.6
0.5
GAIN ERROR (%)
GAIN ERROR (%)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
0
0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
INPUT COMMON MODE (V)
1966 G01
Gain and Offsets vs Temperature
0.5
V
DD
= 2.7V
0.4 V
SS
= GND
0.3
GAIN ERROR (%)
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–50
–25
GAIN ERROR
GAIN ERROR (%)
0.2
V
OOS
–0.4
–0.6
–0.8
NOMINAL
SPECIFIED
CONDITIONS
50
75
25
0
TEMPERATURE (°C)
U W
Gain and Offsets vs Temperature
0.5
0.4
0.5
V
DD
= 5V
0.4 V
SS
= –5V
0.3
OFFSET VOLTAGE (mV)
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
V
IOS
V
OOS
GAIN ERROR
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–50
–25
50
75
25
0
TEMPERATURE (°C)
100
V
IOS
GAIN ERROR
V
OOS
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
125
100
–0.5
125
1966 G08
1966 G09
Gain and Offset
vs Output Common Mode
V
DD
= 2.7V
0.4 V
SS
= GND
0.3
1.0
0.8
V
IOS
0.6
OFFSET VOLTAGE (mV)
OFFSET VOLTAGE (mV)
0.4
0.2
0
V
OOS
–0.2
–0.4
–0.6
–0.8
–1.0
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
0
0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
OUTPUT COMMON MODE (V)
1966 G04
0.4
GAIN ERROR
0.2
0
V
OOS
–0.2
–0.4
–0.6
–0.8
–1.0
Gain and Offset vs V
SS
Supply
1.0
0.8
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
V
DD
= 5V
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
V
IOS
0.6
0.4
0.2
0
–0.2
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–6
–5
–4
–3
V
SS
(V)
GAIN ERROR
V
OOS
V
IOS
0.2
0.1
0
– 0.1
– 0.2
– 0.3
– 0.4
–0.5
100
–1.0
125
–2
–1
0
1966 G11
1966 G07
sn1966 1966fas
5
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