LTC1968
Precision Wide Bandwidth,
RMS-to-DC Converter
FEATURES
■
■
DESCRIPTIO
■
■
■
■
■
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 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
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 1V
PEAK
, 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
APPLICATIO 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
LTC1968,
∆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
1968 TA01b
Single Supply RMS-to-DC Converter
4.5V TO 5.5V
V
+
IN1
DIFFERENTIAL
INPUT
0.1µF
OPT. AC
COUPLING
IN2
EN
OUTPUT
LTC1968
OUT RTN
GND
C
AVE
10µF
+
V
OUT
–
1968 TA01
U
CONVENTIONAL
LOG/ANTILOG
1968f
U
U
1
LTC1968
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)
LTC1968C/LTC1968I ......................... – 40°C to 85°C
Specified Temperature Range (Note 5)
LTC1968C/LTC1968I ......................... – 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
LTC1968CMS8
LTC1968IMS8
MS8 PART MARKING
LTAFG
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.
The temperature grade (I or C) is indicated on the shipping container.
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 Voltage Drift
Linearity Error
Power Supply Rejection
Input Offset Voltage
Input Offset Voltage 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
●
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 20kHz Input (Notes 6, 7)
●
MIN
TYP
±0.1
0.2
MAX
±0.3
±0.4
0.75
10
±0.15
±0.20
±0.25
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)
●
●
●
2
±0.02
±0.02
0.4
(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)
●
●
●
●
●
●
●
●
2
0.2
5
1
0
1.2
100
50
250
1.05
Additional Error vs Crest Factor (CF)
V
+
400
5
700
2
U
V
MΩ
MΩ
µV/V
mV
µV/V
1968f
W
U
U
W W
W
LTC1968
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
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
●
●
The
●
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
●
MIN
0
10
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)
●
●
●
12.5
50
1.05
250
500
15
16
250
(Note 9)
●
1000
µV/V
kHz
MHz
Frequency Response
Power Supplies
4.5
2.3
2.4
0.1
–1
–3
– 0.1
–0.5
2.1
0.1
– 0.1
5.5
2.7
V
mA
mA
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
●
●
●
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 LTC1968 output (V
OUT
) is high impedance and can be
overdriven, either sinking or sourcing current, to the limits stated.
Note 4:
The LTC1968C/LTC1968I are guaranteed functional over the
operating temperature range of – 40°C to 85°C.
Note 5:
The LTC1968C is guaranteed to meet specified performance from
0°C to 70°C. The LTC1968C 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 LTC1968I 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 LTC1968 is 100% tested with C
AVE
= 47nF.
Note 7:
The LTC1968 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 LTC1968 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 LTC1968 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 LTC1968 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 LTC1968 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 LTC1968 are measured
with DC inputs from 50mV to 350mV. The input CMRR is defined as the
change in V
IOS
measured with the input common mode voltage at 0V and
V
+
, divided by V
+
. 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 LTC1968 input and output voltage swings are limited by
internal clipping. However, its
∆Σ
topology is relatively tolerant of
momentary internal clipping.
Note 15:
The LTC1968 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 LTC1968 is inherently wideband, but the output
accuracy is degraded by this aliased noise.
1968f
3
LTC1968
TYPICAL PERFOR A CE CHARACTERISTICS
Gain and Offset
vs Input Common Mode Voltage
0.5
0.4
0.3
GAIN ERROR (%)
50mV
≤
V
IN
≤
350mV
GAIN ERROR (%)
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
V
OOS
V
IOS
GAIN ERROR
–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)
1968 G01
Gain and Offset vs Supply Voltage
0.5
0.4
0.3
50mV
≤
V
IN
≤
350mV
1.0
0.8
0.6
0.5
0.4
0.3
GAIN ERROR (%)
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
4.5
4.8
GAIN ERROR
GAIN ERROR (%)
0.2
5.4
5.7
5.1
SUPPLY VOLTAGE (V)
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
50mV
≤
V
IN
≤
350mV
1.0
0.8
0.6
OFFSET VOLTAGE (mV)
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
GAIN ERROR
0.4
0.2
0
V
OOS
V
IOS
–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)
1968 G02
Gain and Offset vs Temperature
50mV
≤
V
IN
≤
350mV
0.5
0.4
0.3
OFFSET VOLTAGE (mV)
OFFSET VOLTAGE (mV)
V
OOS
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
6.0
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–40
–15
35
10
TEMPERATURE (°C)
60
85
1968 G04
0.2
V
IOS
0.1
0
–0.1
GAIN ERROR
V
OOS
–0.2
–0.3
–0.4
–0.5
V
IOS
1968 G03
1968f
LTC1968
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
4
5
1968 G05
201.0
OUTPUT VOLTAGE (mV DC)
OUTPUT VOLTAGE (mV DC)
20Hz
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
V
OUT
(mV DC) – V
IN
(mV AC
RMS
)
10kHz
60Hz
DC Linearity
0.10
C
AVE
= 10µF
0.08 V
IN2
= MIDSUPPLY
SUPPLY CURRENT (mA)
3.0
2.5
0.06
{V
OUTDC
– |V
INDC
|} (mV)
SUPPLY CURRENT (mA)
0.04
0.02
0
–0.02
–0.04
–0.06 EFFECT OF OFFSETS
–0.08 MAY BE POSITIVE OR
NEGATIVE AT V
IN
= 0V
–0.10
–300
100
–500
–100
V
IN1
(mV)
Power Supply and ENABLE Pin
Current vs ENABLE Voltage
3.0
2.5
SUPPLY CURRENT (mA)
2.0
I
S
1.5
1.0
I
EN
100
0
–100
OUTPUT DC VOLTAGE (mV)
OUTPUT DC VOLTAGE (mV)
0.5
0
–0.5
–1.0
0
1
4
3
5
2
ENABLE PIN VOLTAGE (V)
6
1968 G11
U W
1kHz
300
Performance vs Large Crest Factor
220
210
10kHz
40kHz
1kHz
0.20
20Hz
0.15
0.10
0.05
AC Linearity
SINEWAVES
C
AVE
= 10µF
V
IN2
= MIDSUPPLY
60Hz
0
40kHz
–0.05
–0.10
–0.15
–0.20
0
100
200
300
V
IN1
(mV AC
RMS
)
400
500
1968 G07
7
8
1968 G06
Supply Current vs Supply Voltage
2.44
2.42
2.40
2.38
2.36
2.34
2.32
Supply Current vs Temperature
2.0
1.5
1.0
0.5
0
500
1968 G08
0
1
2
3
4
SUPPLY VOLTAGE (V)
5
6
1968 G09
2.30
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
1968 G10
Input Signal Bandwidth
vs RMS Value
300
200
ENABLE PIN CURRENT (nA)
Input Signal Bandwidth
202
–3dB
200
198
196
194
192
190
188
186
1%/DIV
184 C
AVE
= 10µF
V
IN
= 200mV
RMS
182
1k
10k
100k
1M
100
INPUT SIGNAL FREQUENCY (Hz)
1000
1% ERROR
100
–200
–300
–400
1% ERROR
10
1k
100k
1M
10M
10k
INPUT SIGNAL FREQUENCY (Hz)
100M
1968 G12
10M
1968 G13
1968f
5
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