LTC1992 datasheet(30/40 Pages) LINER

30 / 40 page

LTC1992 Family

1992f

APPLICATIO S I FOR ATIO

The VOCM input pin has a very high input impedance and

is easily driven by even the weakest of sources. Many

ADCs provide a voltage reference output that defines

either its common mode level or its full-scale level. Apply

the ADC’s reference potential either directly to the VOCM

pin or through a resistive voltage divider depending on the

reference voltage’s definition. When controlling the VOCM

pin by a high impedance source, connect a bypass capaci-

tor (1000pF to 0.1µF) from the VOCM pin to ground to lower

the high frequency impedance and limit external noise

coupling. Other applications will want the output biased at

a midpoint of the power supplies for maximum output

voltage swing. For these applications, the LTC1992 pro-

vides a midsupply potential at the VMID pin. The VMID pin

connects to a simple resistive voltage divider with two

200k resistors connected between the supply pins. To use

this feature, connect the VMID pin to the VOCM pin and

bypass this node with a capacitor.

One undesired effect of utilizing the level shifting function

is an increase in the differential output offset voltage due

to gain setting resistor mismatch. The offset is approxi-

mately the amount of level shift (VOUTCM – VINCM) multi-

plied by the amount of resistor mismatch. For example, a

2V level shift with 0.1% resistors will give around 2mV of

output offset (2 • 0.1% = 2mV). The exact amount of offset

is dependent on the application’s gain and the resistor

mismatch. For a detail description, consult the Fully Differ-

ential Amplifier Applications Circuit Analysis section.

CMRR and Output Balance

One common misconception of fully differential amplifiers

is that the common mode level servo guarantees an

infinite common mode rejection ratio (CMRR). This is not

true. The common mode level servo does, however, force

the two outputs to be truly complementary (i.e., exactly

opposite or 180 degrees out of phase). Output balance is

a measure of how complementary the two outputs are.

At low frequencies, CMRR is primarily determined by the

matching of the gain setting resistors. Like any op amp,

the LTC1992 does not have infinite CMRR, however resis-

tor mismatching of only 0.018%, halves the circuit’s

CMRR. Standard 1% tolerance resistors yield a CMRR of

about 40dB. For most applications, resistor matching

dominates low frequency CMRR performance. The speci-

fications for the fixed gain LTC1992-X parts include the

on-chip resistor matching effects. Also, note that an

input

common mode signal appears as a differential output

signal reduced by the CMRR. As with op amps, at higher

frequencies the CMRR degrades. Refer to the Typical

Performance plots for the details of the CMRR perfor-

mance over frequency.

At low frequencies, the output balance specification is

determined by the matching of the on-chip RCMM and

RCMP resistors. At higher frequencies, the output balance

degrades. Refer to the typical performance plots for the

details of the output balance performance over frequency.

Input Impedance

The input impedance for a fully differential amplifier appli-

cation circuit is similar to that of a standard op amp

inverting amplifier. One major difference is that the input

impedance is different for differential input signals and

single-ended signals. Referring to Figure 3, for differential

input signals the input impedance is expressed by the

following expression:

RINDIFF = 2 • RIN

For single-ended signals, the input impedance is ex-

pressed by the following expression:

INS

-E =

()

–

•

The input impedance for single-ended signals is slightly

higher than the RIN value since some of the input signal is

fed back and appears as the amplifier’s input common

mode level. This small amount of positive feedback in-

creases the input impedance.

Driving Capacitive Loads

The LTC1992 family of parts is stable for all capacitive

loads up to at least 10,000pF. While stability is guaranteed,

the part’s performance is not unaffected by capacitive

loading. Large capacitive loads increase output step re-

sponse ringing and settling time, decrease the bandwidth

LTC1992 Datasheet(PDF) 30 Page - Linear Technology

LTC1992 Datasheet(HTML) 30 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL