LTC1966HMS8-PBF View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1966HMS8-PBF

Precision Micropower ∆∑ RMS-to-DC Converter

Linear Technology 

LTC1966

Applications Information

However, resistive loading is an issue and the 10MΩ

impedance of a DMM or 10× scope probe will drag the

output down by –0.85% typ.

During shutdown, the switching action is halted and a

fixed 30k resistor shunts VOUT to OUT RTN so that CAVE

is discharged.

Guard Ringing the Output

The LTC1966’s combination of precision and high output

impedance can present challenges that make the use of

a guard ring around the output a good idea for many ap-

plications.

As mentioned above, a 10M resistive loading to ground

will drag down the gain far more than the specificed gain

tolerance. On a printed circuit board, contaminants from

solder flux residue to finger grime can create parasitic

resistances, which may be very high impedance, but can

have deleterious effects on the realized accuracy. As an

example, if the output (Pin 5) is routed near VSS (Pin 4)

in a ±5V application, a parasitic resistance of 1G (1,000M)

is enough to introduce a –425µV output offset error, more

than the specified limit of the LTC1966 itself.

Use of a guard ring, wherein the LTC1966 output node is

completely surrounded by a low impedance voltage, can

reduce leakage related errors substantially. The ground

ring can be tied to OUTRTN (Pin 6) and should encircle the

output (Pin 5), the averaging capacitor terminal, and the

destination terminal at the ADC, filter op amp, or whatever

else may be next.

Figure 24a shows a sample PCB layout for the circuit of

Figure 13, wherein the guard ring trace encloses R1, R2,

and the terminals of C1, C2, and the op amp input con-

nected to the high impedance LTC1966 Output. For the

circuit of figure 14, the guard ring should enclose R1 and

the terminals of C1 and C2, as well as the terminal at the

ultimate destination.

Figure 24b shows a sample PCB layout for the circuit of

Figure 23. The summing node of the LT1494 has the same

high impedance and high accuracy as the LTC1966 output,

so here the guard ring encircles both of them. Any leakage

between them is benign because the LT1494 forces them

to the same nominal voltage.

LTC1966

MS8

0.1µF

LTC1966

MS8

CAVE 1µF

LT1494 1966 F24b

SO8

CAVE 1µF

0.1µF

1966 F24a

LT1880

SO8

Figure 24a. PCB Layout of Figure 13 with Guard Ring

Figure 24b. PCB Layout of Figure 23 with Guard Ring

1966fb

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