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LTC1968

1968f

Input Connections

The LTC1968 input is differential and DC coupled. The

LTC1968 responds to the RMS value of the differential

voltage between Pin 2 and Pin 3, including the DC portion

of that difference. However, there is no DC-coupled path

from the inputs to ground. Therefore, at least one of the two

inputs must be connected with a DC-return path to ground.

Both inputs must be connected to something. If either

input is left floating, a zero volt output will result.

For single-ended DC-coupled applications, simply con-

nect one of the two inputs (they are interchangeable) to

the signal, and the other to ground. This will work well for

dual supply configurations, but for single supply con-

figurations it will only work well for unipolar input sig-

nals. The LTC1968 input voltage range is from rail-to-rail,

and when the input is driven above V+ or below GND the

gain and offset errors will increase substantially after just

a few hundred millivolts of overdrive. Fortunately, most

single supply circuits measuring a DC-coupled RMS

value will include some reference voltage other than

ground, and the second LTC1968 input can be connected

to that point.

For single-ended AC-coupled applications, Figure 9 shows

three alternate topologies. The first one, shown in Figure

9a uses a coupling capacitor to one input while the other

is grounded. This will remove the DC voltage difference from

the input to the LTC1968, and it will therefore not be part

of the resulting output voltage. Again, this connection will

APPLICATIO S I FOR ATIO

work well with dual supply configurations, but in single

supply configurations it will be necessary to raise the volt-

age on the grounded input to assure that the signal at the

active input stays within the range of 0V to V+. If there is

already a suitable voltage reference available, connect the

second input to that point. If not, a midsupply voltage can

be created with two resistors as shown in Figure 9b.

Finally, if the input voltage is known to be between 0V and

V+, it can be AC coupled by using the configuration shown

in Figure 9c. Whereas the DC return path was provided

through Pin 3 in Figures 9a and 9b, in this case, the return

path is provided on Pin 2, through the input signal volt-

ages. The switched capacitor action between the two input

pins of the LTC1968 will cause the voltage on the coupling

capacitor connected to the second input to follow the DC

average of the input voltage.

For differential input applications, connect the two inputs to

the differential signal. If AC coupling is desired, one of the

two inputs can be connected through a series capacitor.

In all of these connections, to choose the input coupling

capacitor, CC, calculate the low frequency coupling time

constant desired, and divide by the LTC1968 differential

input impedance. Because the LTC1968 input impedance

is about 100 times its output impedance, this capacitor is

typically much smaller than the output averaging capaci-

tor. Its requirements are also much less stringent, and a

ceramic chip capacitor will usually suffice.

Figure 9. Single-Ended AC-Coupled Input Connection Alternatives

–

LTC1968

GND

V+

V–

(9a)

IN1

VIN

IN2

LTC1968

(9b)

10k

0.1µF

IN1

VIN

V+

IN2

LTC1968

(9c)

IN1

VIN

1968 F09

VDC

IN2

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