AD8250 datasheet(21/34 Pages) AD

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LTC6373

Rev. 0

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Output Common Mode and VOCM Pin

The output common mode voltage is defined as the aver-

age of the two outputs:

VOUTCM = (V+OUT + V–OUT)/2 = VOCM

As the equation shows, the output common mode voltage

is independent of the input common mode voltage, and

is instead determined by the voltage on the VOCM pin, by

means of an internal common mode feedback loop. If

the VOCM pin is left floating, an internal resistor divider

creates a default voltage approximately halfway between

V+OUT and V–. The VOCM pin can be overdriven to another

voltage if desired for greater accuracy or flexibility. For

example, when driving an ADC, if the ADC makes a refer-

ence available for setting the common mode voltage, it

can be directly tied to the VOCM pin, as long as the ADC is

capable of driving the 2.3MΩ input resistance presented

by the VOCM pin. The Electrical Characteristics table speci-

fies the valid range that can be applied to the VOCM pin

(VOUTCMR).

Input Pin Protection

To prevent damage, the LTC6373 has a comprehensive

protection scheme, especially on the input pins, as illus-

trated in the Simplified Block Diagram of Figure 1. The

input current applied to the LTC6373’s input pins should

be kept under ±10mA. To achieve additional input protec-

tion, external series resistors and/or low leakage clamp

diodes should be used.

Reducing Board-Related Leakage Effects

Leakage currents can have a significant impact on sys-

tem accuracy, particularly in high temperature and high

voltage applications. Quality insulation materials should

be used, and insulating surfaces should be cleaned to

remove fluxes and other residues. For humid environ-

ments, surface coating may be necessary to provide a

moisture barrier.

6373 F04

LTC6373

–IN

+IN

Figure 4. Guard Rings Can Be Used to Minimize

Leakage into the Input Pins

Leakage into the input pins reacts with the source resis-

tance, creating an error directly at the input. As shown in

Figure 4, this leakage can be minimized by enclosing the

input connections with guard rings operated at a potential

very close to that of the input pins. For the lowest leakage,

amplifiers can be used to drive the guard rings. These

buffers must have very low input bias current since that

current will now be a leakage current.

Input Bias Current Return Path

The low input bias current (25pA max) and high input

impedance (5000GΩ) of the LTC6373 allow the use of

high impedance sources without introducing additional

offset voltage errors, even when the full common mode

range is required. However, a DC path must be provided

for the input bias currents of both inputs when a purely

differential signal is being amplified. Without this path, the

inputs will float to either rail and exceed the input volt-

age range of the LTC6373, resulting in a saturated input

amplifier. Figure 5 shows three examples of an input bias

current path. The first example is of a purely differential

signal source with a 10kΩ input current path to ground.

Since the impedance of the signal source is low, only one

resistor is needed. Two matching resistors are needed for

higher impedance signal sources as shown in the second

example. Balancing the input impedance improves both

DC and AC common mode rejection as well as DC offset.

The need for input resistors is eliminated if a center tap

is present as shown in the third example.

APPLICATIONS INFORMATION

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