QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B
RMS TO DC CONVERTER
3
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.
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-
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:
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.
