QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B
RMS TO DC CONVERTER
LTC1966/LTC1967/LTC1968
DESCRIPTION
Demonstration circuit 427B features the LTC1966/7/8 Pre-
cision Micro-power true RMS-to-DC converter. It incorpo-
rates a variety of passive components to support configu-
rations for varied applications. Two optional post-filters
are included to improve averaging ripple and step re-
sponse performance, as illustrated in the Design Cook-
book section of the datasheet. DC427B-A contains the
LTC1966, DC427B-B contains the LTC1967, and DC427B-
C contains the LTC1968.
The LTC1966/7/8 is a patent-pending
∆Σ-based
RMS to
DC converter that is simpler, more accurate, more flexible,
and lower power than conventional log-antilog RMS-to-DC
converters. The LTC1966/7/8 accepts single-ended or dif-
ferential inputs with rail-to-rail common-mode range. One
standout feature of the LTC1966/7/8 is its superior linear-
ity, which allows hassle-free system calibration at any in-
put voltage.
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
QUICK START PROCEDURE
Table 1 shows the Jumper positions and their respective
effects. Be sure that the jumpers are in the
bold
(default)
positions. Refer to Figure 1 for the connection diagram
and follow the procedure below:
1.
Connect the power supplies as shown. The power labels
Table 1: DC427B Jumpers (default positions in bold)
JUMPER
JP1
SETTING
Top
Bottom
Removed
JP2
Left
Right
JP3
Top
Right
Bottom
JP4
Left
Right
JP5
Top
Bottom
Removed
JP6
Left
Right
EFFECT
LTC1966/7/8 shutdown
LTC1966/7/8 always enabled
LTC1966/7/8 controlled by E2 input
to DC427
IN1 grounded
IN1 active (J1 connected to pin 2)
Vout (pin 5) connected to optional
DC-Accurate post-filter
Vout connected to output terminal
E4, no filter
Vout connected to optional buffered
post-filter
IN2 grounded
IN2 active (J2 connected to pin 3)
OutRtn (pin 6) grounded
OutRtn connected to VSS
OutRtn floating; drive with voltage at
terminal E7
LTC1966/7/8 negative supply (Pin
1) connected to VSS
Pin 1 grounded
of V
SS
and V
DD
directly correspond to pins 4 and 7 of the
LTC1966/7/8, respectively, and combinations other than
±5V
may be used. The LTC1966 accepts dual
±5V
sup-
plies, and the LTC1967/8 accept single 5V supplies; re-
fer to the respective datasheet and the Supply Voltages
section of this Quick Start Guide.
2.
Apply an input signal to convert from RMS to DC to
IN1. Any signal generator may be used; the LTC1966
works up to 1V
PEAK
with V
DD
= 5V.
NOTE:
There is a 50Ω termination to ground on DC427B;
remove if driving IN1 or IN2 with an op-amp.
3.
Observe the output with a digital voltmeter (DVM) con-
nected to terminals E4 and E7.
NOTE:
The LTC1966/7/8 output impedance is high, and
10MΩ DVM input impedance will cause up to –
1% gain error.
4.
To use the optional output filters, reposition JP3 and
observe the output at terminal E1 or E8.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B
RMS TO DC CONVERTER
Note:
LTC1967 and LTC1968 run on single 5V supplies. See
Supply Voltages section (below) and product datasheet.
Figure 1.
Proper Measurement Equipment Setup
ADDITIONAL FEATURES
Although the 427B demo board is ready to use out of the
box (See “Quick Start Procedure” above), it has features
that you can access by adding, removing or changing
components on the board. Here are some ways that you
can change the configuration to take advantage of these
features.
DIFFERENTIAL INPUTS
To drive the input differentially, move jumper JP4 from left
to right. The LTC1966/7/8 will respond to the difference of
the signals. If two different frequencies are used, the vec-
tor sum results.
AVERAGING CAPACITORS
The board provides space for trying out different averaging
(and filtering) capacitors. The space at C7B parallels C7A
(the included metal film averaging capacitor). The space
has a footprint that will accommodate 2.5mm, 5mm,
7.5mm, and 10mm spaced through-hole leaded compo-
nents as well as a large solder mask-free area for use with
a wide variety of surface mount case sizes. Try out the
averaging capacitor you intend to use with the
LTC1966/7/8 in 427B.
SUPPLY VOLTAGES
The LTC1967/8 are single 5V supply converters. V
DD
and
V
SS
on the DC427B connect to Pin 7 and Pin 4 of the IC,
but the supply pins of the LTC1967/8 are Pin 7 and Pin 1
(Pin 4 is NC). On the DC427B-B and DC427B-C, use JP6
to select whether the negative supply voltage of the
LTC1967/8 will be VSS or ground.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B
RMS TO DC CONVERTER
FILTER OP AMPS
Likewise, the layout of the U2/U3 Op Amps includes active
locations for SOT-23, MS-8, SO-8 and DIP-8 single Op
Amp packages. Try out the Op Amp you intend to use in
such filters using DC427B. Normally unused pins of the
Op Amps (1, 5, and 8 in the 8-lead packages) are all tied in
parallel, i.e. pin 8 of the DIP is tied to pin 8 of the SO and
pin 8 of the MS. Pin 5 of the SOT-6 is tied to pin 5 of the
other packages as well. If using any of these pins (for
shutdown control, nulling, compensation, etc) the DIP-8
through-hole pins will likely be the easiest place to con-
nect to them.
NOTE:
The MS-8 and DIP-8 are located on the back (solder)
side of the board. Using the back side for these parts
is not advised for production.
sitics. In a 50Ω environment, even 0.05Ω in series will
result in a 0.1% attenuation of the signal. Of course many
50Ω terminators are
±1%
tolerance, including R7 & R9 on
DC427B. To compensate for these effects, the signal at the
LTC1966 inputs can be Kelvin-sensed. But parasitic ca-
pacitance can corrupt the LTC1966 sampling or the
LTC1966 sampling can reduce the sensed signal level, de-
pending on the exact impedance and parasitic levels. The
preferred method is to sense at the outside of series feed
resistors R6 and R8, at the test points TP1 and TP2 lo-
cated to the left of R6 and R8. For a further discussion of
the LTC1966/7/8 input sampling, refer to the “Input Im-
pedance” information in the “Application Information” sec-
tion of the datasheet.
JACKS AND TURRETS
Connectors J1 and J2 allow installation of a BNC jack,
such as CONNEX type 112404.
The various holes around the periphery of the board allow
the installation of turrets such as MILL-MAX type 2501-2.
However, the size and placement of the holes allow clip
lead connections without installing the turrets.
SHUTDOWN
Finally, by removing jumper JP1, the LTC1966/7/8 ENABLE
pin may be driven externally through E2.
NOTE:
AC COUPLING
Resistors R6 and R8 put 10kΩ between the LTC1966/7/8
and the signals driving DC427B. With 50Ω signal genera-
tors, these are not needed, but they are helpful when driv-
ing DC427 with standard Op Amps whose output stages
can get confused by the fast spikes of current drawn by
the LTC1966/7/8's CMOS inputs (described in the data-
sheet). Another important reason for including R6/R8 is
the ability to change one of them to a capacitor to provide
AC coupling. Select a 1206 case size capacitor as de-
scribed in the LTC1966/7/8 datasheet.
KELVIN-SENSE POINTS
The LTC1966/7/8 has a typical conversion gain accuracy of
±0.1%.
Measuring this accuracy can be difficult. The pri-
mary challenge is getting a known good accurate input
signal to the LTC1966/7/8 without corrupting it with para-
The LT1494 Op-Amps and the R1/R3 divider network
will continue to draw current even when the
LTC1966/7/8 is disabled. To measure the near-zero
current of the LTC1966, remove zero-Ω shunts R14
and R15. These can also be removed to power the
optional filters with different supply voltages than the
LTC1966/7/8.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B
RMS TO DC CONVERTER
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