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LTC6912

6912fa

AGND: Analog Ground. The AGND pin is at the midpoint of

an internal resistive voltage divider, developing a potential

halfway between the V+ and V– pins. In normal operation,

the AGND pin has an equivalent input resistance of nomi-

nally 50k (Figure 1). In order to reduce the quiescent

supply current in hardware shutdown (SHDN pin pulled to

V+, GN-16 only), the equivalent series resistance of this

pin significantly increases (to a value on the order of

800k

Ω with 5V supplies, but is highly supply voltage,

temperature, and process dependent). AGND is the

noninverting input to both the internal channel A and

channel B amplifiers. This makes AGND the ground refer-

ence voltage for the INA, INB, OUTA, and OUTB pins.

Recommended analog ground plane connection depends

on how power is applied to the LTC6912-X (See Figures 2,

3, and 4). Single power supply applications typically use

V– for the system signal ground. The analog ground plane

in single-supply applications should therefore tie to V–,

and the AGND pin should be bypassed to this ground plane

by a high quality capacitor of at least 0.1

µF (Figure 2). The

AGND pin provides an internal analog reference voltage at

half the V+ supply voltage. Dual supply applications with

symmetrical supplies (such as

±5V) have a natural system

ground plane potential of zero volts, in which the AGND pin

can be directly tied to, making the zero volt ground plane

the input and output reference voltage for the LTC6912-X

(Figure 3). Finally, if dual asymmetrical power supplies are

used, the supply ground is still the natural ground plane

voltage. To maximize signal swing capability with an

asymmetrical supply, however, it is often desirable to refer

the LTC6912-X’s analog input and output to a voltage

equidistant from the two supply rails V+ and V–. The AGND

pin will provide such a potential when open-circuited and

bypassed with a capacitor (Figure 4). In noise sensitive

applications where AGND does not tie directly to a ground

plane, as in Figures 2 and 4, it is important to AC-bypass

the AGND pin. Otherwise channel to channel isolation is

degraded, and wideband noise will enter the signal path

from the thermal noise of the internal voltage divider

resistors which present a Thévenin equivalent resistance

of approximately 50k

Ω. This noise can reduce SNR by at

least 15dB at high gain settings. An external capacitor

from AGND to the ground plane, whose impedance is well

below 50k

Ω at frequencies of interest, will filter and

suppress this noise. A 0.1

µF high quality capacitor is

effective for frequencies down to 1kHz. Larger capacitors

will extend this suppression to lower frequencies. This

issue does not arise in dual supply applications because

the AGND pin ties directly to ground. In applications

requiring an analog ground reference other than half the

total supply voltage, the user can override the built-in

analog ground reference by tying the AGND pin to a

reference voltage with the AGND voltage range specified in

the Electrical Characteristics Table. The AGND pin will load

the external reference with approximately 50k

Ω returned

to the half-supply potential. AGND should still be capaci-

tively bypassed to a ground plane as noted above. Do not

connect the AGND pin to the V– pin.

Figure 2. Single Supply Ground Plane Connection

LTC6912-X

SERIAL

INTERFACE

0.1

µF

V+

DIGITAL GROUND PLANE

ANALOG GROUND PLANE

SINGLE-POINT

SYSTEM GND

≥0.1µF

V+

REFERENCE

6912 F02

PI FU CTIO S

Figure 3. Symmetrical Dual Supply Ground Plane Connection

LTC6912-X

SERIAL

INTERFACE

0.1

µF

0.1

µF

V+

V–

DIGITAL GROUND PLANE

ANALOG GROUND PLANE

6912 F03

SINGLE-POINT

SYSTEM GND

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