FEATURES
s
s
LTC1967
Precision Extended
Bandwidth, RMS-to-DC Converter
DESCRIPTIO
The LTC
®
1967 is a true RMS-to-DC converter that uses an
innovative delta-sigma computational technique. The ben-
efits of the LTC1967 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 LTC1967 operates with single-ended or differential in-
put signals (for EMI/RFI rejection) and supports crest fac-
tors up to 4. Common mode input range is rail-to-rail. Dif-
ferential input range is 1V
PEAK
, and offers unprecedented
linearity. The LTC1967 allows hassle-free system calibra-
tion at any input voltage.
The LTC1967 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 LTC1967 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
s
s
s
s
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High Linearity:
0.02% Linearity Allows Simple System Calibration
Wide Input Bandwidth:
Bandwidth to 0.1% Additional Gain Error: 40kHz
Bandwidth Independent of Input Voltage Amplitude
No-Hassle Simplicity:
True RMS-DC Conversion with Only One External
Capacitor
Delta Sigma Conversion Technology
Low Supply Current:
330µA Typ
Ultralow Shutdown Current:
0.1µA
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
APPLICATIO S
s
s
True RMS Digital Multimeters and Panel Meters
True RMS AC + DC Measurements
TYPICAL APPLICATIO
Linearity Performance
LINEARITY ERROR (V
OUT
mV DC – V
IN
mV AC
RMS
)
0.2
LTC1967,
∆Σ
0
–0.2
–0.4
–0.6
–0.8
–1.0
60Hz SINEWAVE
0
100
200
300
V
IN
(mV AC
RMS
)
400
500
1967 TA01b
Single Supply RMS-to-DC Converter
4.5V TO 5.5V
V
+
IN1
DIFFERENTIAL
INPUT
0.1µF
OPT. AC
COUPLING
IN2
EN
OUTPUT
LTC1967
OUT RTN
GND
C
AVE
1µF
+
V
OUT
–
1967 TA01
U
U
U
CONVENTIONAL
LOG/ANTILOG
1967f
1
LTC1967
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
GND
IN1
IN2
NC
1
2
3
4
8
7
6
5
ENABLE
V
+
OUT RTN
V
OUT
Supply Voltage
V
+
to GND ............................................................. 6V
Input Currents (Note 2) .....................................
±10mA
Output Current (Note 3) .....................................
±10mA
ENABLE Voltage ......................................... –0.3V to 6V
OUT RTN Voltage ........................................ –0.3V to V
+
Operating Temperature Range (Note 4)
LTC1967C/LTC1967I ......................... – 40°C to 85°C
Specified Temperature Range (Note 5)
LTC1967C/LTC1967I ......................... – 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
LTC1967CMS8
LTC1967IMS8
MS8 PART MARKING
LTTJ
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
∆V
OOS
/∆T
LIN
ERR
PSRRG
V
IOS
∆V
IOS
/∆T
PARAMETER
Low Frequency Gain Error
Output Offset Voltage
Output Offset Drift
Linearity Error
Power Supply Rejection
Input Offset Voltage
Input Offset Drift
CF = 3
CF = 5
Input Characteristics
V
IMAX
I
VR
Z
IN
CMRRI
V
IMIN
PSRRI
Maximum Peak Input Swing
Input Voltage Range
Input Impedance
Input Common Mode Rejection
Minimum RMS Input
Power Supply Rejection
Conversion Accuracy
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
+
= 5V, V
OUTRTN
= 2.5V, C
AVE
= 10µF, V
IN
= 200mV
RMS
, V
ENABLE
= 0.5V
unless otherwise noted.
CONDITIONS
50Hz to 5kHz Input (Notes 6, 7)
q
MIN
TYP
±0.1
0.1
MAX
±0.3
±0.4
0.55
10
0.15
0.15
0.20
1.5
10
UNITS
%
%
mV
µV/°C
%
%/V
%/V
mV
µV/°C
mV
mV
V
(Notes 6, 7)
(Note 11)
50mV to 350mV (Notes 7, 8)
(Note 9)
q
q
q
2
0.02
0.02
0.2
(Notes 6, 7, 10)
(Note 11)
60Hz Fundamental, 200mV
RMS
60Hz Fundamental, 200mV
RMS
Accuracy = 1% (Note 14)
Average, Differential (Note 12)
Average, Common Mode (Note 12)
(Note 13)
(Note 9)
q
q
q
q
1
0.2
5
1
0
5
100
50
250
1.05
Additional Error vs Crest Factor (CF)
q
q
q
q
V
+
400
5
600
2
U
V
MΩ
MΩ
µV/V
mV
µV/V
1967f
W
U
U
W W
W
LTC1967
ELECTRICAL CHARACTERISTICS
SYMBOL
OVR
Z
OUT
CMRRO
V
OMAX
PSRRO
f
1P
f
– 3dB
V
+
I
S
PARAMETER
Output Voltage Range
Output Impedance
Output Common Mode Rejection
Maximum Differential Output Swing
Power Supply Rejection
0.1% Additional Gain Error (Note 15)
±3dB
Frequency (Note 15)
Supply Voltage
Supply Current
IN1 = 20mV, IN2 = 0V
IN1 = 200mV, IN2 = 0V
V
ENABLE
= 4.5V
V
ENABLE
= 4.5V
V
ENABLE
= 0.5V
q
q
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are T
A
= 25°C. V
+
= 5V, V
OUTRTN
= 2.5V, C
AVE
= 10µF, V
IN
= 200mV
RMS
, V
ENABLE
= 0.5V
unless otherwise noted.
CONDITIONS
q
MIN
0
40
1.0
0.9
TYP
MAX
V
+
UNITS
V
kΩ
µV/V
V
V
Output Characteristics
(Note 12)
(Note 13)
Accuracy = 1%, DC Input (Note 14)
q
q
q
50
50
1.05
250
40
4
65
250
(Note 9)
q
1000
µV/V
kHz
MHz
Frequency Response
Power Supplies
4.5
320
340
0.1
–1
–3
– 0.1
–0.5
2.1
0.1
– 0.1
5.5
390
V
µA
µA
µA
µA
µA
V
V
Shutdown Characteristics
I
SS
I
IH
I
IL
V
TH
V
HYS
Supply Current
ENABLE Pin Current High
ENABLE Pin Current Low
ENABLE Threshold Voltage
ENABLE Threshold Hysteresis
q
q
q
10
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 GND and
V
+
. If the inputs are driven beyond the rails, the current should be limited
to less than 10mA.
Note 3:
The LTC1967 output (V
OUT
) is high impedance and can be
overdriven, either sinking or sourcing current, to the limits stated.
Note 4:
The LTC1967C/LTC1967I are guaranteed functional over the
operating temperature range of – 40°C to 85°C.
Note 5:
The LTC1967C is guaranteed to meet specified performance from
0°C to 70°C. The LTC1967C 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 LTC1967I 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 LTC1967 is 100% tested with C
AVE
= 47nF. Correlation
tests have shown that the performance limits 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 LTC1967 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 can be guaranteed with the
additional testing being performed to guarantee proper operation of all
internal circuitry.
Note 8:
The LTC1967 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 LTC1967 are measured with
DC inputs from 50mV to 350mV. The change in accuracy from V
+
= 4.5V
to V
+
= 5.5V is divided by 1V.
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 LTC1967 behavior is simpler to characterize and the input offset
voltage is the only significant source of “DC reversal error.”
Note 11:
Guaranteed by design.
Note 12:
The LTC1967 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.
1967f
3
LTC1967
ELECTRICAL CHARACTERISTICS
Note 13:
The common mode rejection ratios of the LTC1967 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 0V to 350mV and input
levels of V
+
– 350mV to V
+
divided by V
+
– 350mV. The output CMRR is
defined as the change in V
OOS
measured with OUT RTN = 0V and OUT RTN
= V
+
– 350mV divided by V
+
– 350mV.
Note 14:
The LTC1967 input and output voltage swings are limited by
internal clipping. However, its
∆Σ
topology is relatively tolerant of
momentary internal clipping.
Note 15:
The LTC1967 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 LTC1967 is inherently wideband, but the output
accuracy is degraded by this aliased noise.
TYPICAL PERFOR A CE CHARACTERISTICS
Gain and Offset
vs Input Common Mode Voltage
0.3
0.2
0.1
GAIN ERROR (%)
50mV
≤
V
IN(PEAK)
≤
350mV
GAIN ERROR
V
IOS
GAIN ERROR (%)
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–1.0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
INPUT COMMON MODE VOLTAGE (V)
1967 G01
Gain and Offsets vs Temperature
0.05
0.04
0.03
50mV
≤
V
IN(PEAK)
≤
350mV
0.5
0.4
0.3
GAIN ERROR (%)
GAIN ERROR (%)
0.02
0.01
0
–0.01
–0.02
–0.03
–0.04
–0.05
–40
–15
GAIN ERROR
35
10
TEMPERATURE (°C)
4
U W
Gain and Offset
vs Output Common Mode Voltage
1.0
0.8
0.6
OFFSET VOLTAGE (mV)
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
V
IOS
V
OOS
50mV
≤
V
IN(PEAK)
≤
350mV
GAIN ERROR
1.0
0.8
0.6
OFFSET VOLTAGE (mV)
0.4
0.2
0
V
OOS
–0.2
–0.4
–0.6
–0.8
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
OUTPUT COMMON MODE VOLTAGE (V)
1967 G02
Gain and Offset vs Supply Voltage
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
50mV
≤
V
IN(PEAK)
≤
350mV
1.0
0.8
0.6
OFFSET VOLTAGE (mV)
V
OOS
0.2
0.1
0
–0.1
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
4.5
4.8
GAIN ERROR
V
OOS
V
IOS
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
V
IOS
–0.2
–0.3
–0.4
60
85
1967 G03
–0.5
5.7
5.4
5.1
SUPPLY VOLTAGE (V)
–1.0
6.0
1967 G04
1967f
LTC1967
TYPICAL PERFOR A CE CHARACTERISTICS
Performance vs Crest Factor
200mV
RMS
SCR WAVEFORMS
200.8 C
AVE
= 10µF
O.1%/DIV
200.6
200.4
200.2
200.0
199.8
199.6
199.4
199.2
199.0
1
2
3
CREST FACTOR
20Hz
60Hz
1kHz
201.0
220
210
OUTPUT VOLTAGE (mV DC)
OUTPUT VOLTAGE (mV DC)
200
190
180
170
160
150
140
200mV
RMS
SCR WAVEFORMS
130 C
AVE
= 10µF
5%/DIV
120
6
2
3
5
4
1
CREST FACTOR
60Hz
10kHz
V
OUT
(mV DC) – V
IN
(mV AC
RMS
)
DC Linearity
C
AVE
= 1µF
0.08 V
IN2
= MIDSUPPLY
0.06
{V
OUTDC
– |V
INDC
|} (mV)
0.10
450
400
SUPPLY CURRENT (µA)
0.04
0.02
0
–0.02
–0.04
–0.06 EFFECTS OF OFFSETS
–0.08 MAY BE POSITIVE OR
NEGATIVE AT V
IN
= 0V
–0.10
–300
100
–500
–100
V
IN1
(mV)
SUPPLY CURRENT (µA)
Shutdown Current
vs ENABLE Voltage
500
400
SUPPLY CURRENT (µA)
I
S
300
I
EN
200
100
0
–100
OUTPUT DC VOLTAGE (mV)
OUTPUT DC VOLTAGE (mV)
100
0
–100
0
1
4
3
5
2
ENABLE PIN VOLTAGE (V)
6
1967 G11
U W
4
300
1967 G08
Performance vs Large Crest Factors
0.20
AC Linearity
0.15
0.10
0.05
0
60Hz SINEWAVES
C
AVE
= 10µF
V
IN2
= MIDSUPPLY
10Hz
20Hz
1kHz
–0.05
–0.10
–0.15
–0.20
0
100
200
300
V
IN1
(mV AC
RMS
)
400
500
1967 G07
5
1967 G05
7
8
1967 G06
Supply Current vs Supply Voltage
345
340
350
300
250
200
150
100
50
0
500
0
1
2
4
3
SUPPLY VOLTAGE (V)
5
6
1967 G09
Supply Current vs Temperature
V
S
= 5V
335
330
325
320
315
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1967 G10
Input Signal Bandwidth
vs RMS Value
300
200
ENABLE PIN CURRENT (nA)
Input Signal Bandwidth
10%
ERROR
1000
0.1% ERROR
1% ERROR
202
200
198
196
194
192
190
188
186
184 1%/DIV
C
AVE
= 1µF
182
1k
10k
100k
1M
100
INPUT SIGNAL FREQUENCY (Hz)
100
–200
–300
–400
10
–3dB
1
100
1k
10k
100k
1M
INPUT SIGNAL FREQUENCY (Hz)
10M
1967 G12
10M
1967 G13
1967f
5
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