Linear Technology LTC1966 Manuale utente

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:

Connect the power supplies as shown. The power labels

of V

and V

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.

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

= 5V.

NOTE:

There is a 50Ωtermination to ground on DC427B;

remove if driving IN1 or IN2 with an op-amp.

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

10MΩDVM input impedance will cause up to –

1% gain error.

To use the optional output filters, reposition JP3 and

observe the output at terminal E1 or E8.

Table 1: DC427B Jumpers (default positions in bold)

JUMPER SETTING EFFECT

Top LTC1966/7/8 shutdown

Bottom LTC1966/7/8 always enabled

JP1

Removed LTC1966/7/8 controlled by E2 input

to DC427

Left IN1 groundedJP2

Right IN1 active (J1 connected to pin 2)

Top Vout (pin 5) connected to optional

DC-Accurate post-filter

Right Vout connected to output terminal

E4, no filter

JP3

Bottom Vout connected to optional buffered

post-filter

Left IN2 groundedJP4

Right IN2 active (J2 connected to pin 3)

Top OutRtn (pin 6) grounded

Bottom OutRtn connected to VSS

JP5

Removed OutRtn floating; drive with voltage at

terminal E7

Left LTC1966/7/8 negative supply (Pin

1) connected to VSS

JP6

Right Pin 1 grounded

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B

RMS TO DC CONVERTER

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

and

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.

Figure 1. Proper Measurement Equipment Setup

Note

: LTC1967 and LTC1968 run on single 5V supplies. See

Supply Voltages section (below) and product datasheet.

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.

AC COUPLING

Resistors R6 and R8 put 10kΩbetween the LTC1966/7/8

and the signals driving DC427B. With 50Ωsignal 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 50Ωenvironment, even 0.05Ωin series will

result in a 0.1% attenuation of the signal. Of course many

50Ωterminators 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-Ωshunts R14

and R15. These can also be removed to power the

optional filters with different supply voltages than the

LTC1966/7/8.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 427B

RMS TO DC CONVERTER

Mouser Electronics

Authorized Distributor

Click to View Pricing, Inventory, Delivery & Lifecycle Information:

Analog Devices Inc.:

DC427B-C DC427B-A DC427B-B

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