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LTC6373

Rev. 0

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APPLICATIONS INFORMATION

Diamond Plot Interpretation

Diamond plots can be used to determine the valid input

common mode voltage (VICM) operating range for instru-

mentation amplifiers such as LTC6373. The valid region

of operation is where all signals, input or output, are

not clipped.

Subplots (a)-(g) of Figure 2 show the input common

mode voltage (VICM) range allowed for a given differential

output voltage (VOUTDIFF), under various combinations of

gain (G) and supply (VS) settings. In each plot, the output

stage positive supply pin V+OUT is tied to the main positive

supply pin V+, VOCM = 0V (mid-rail) and there is no load.

To identify the valid VICM range for a specific applica-

tion: First, identify the gain and supply conditions that the

LTC6373 will be operated under. Then, identify the range

of valid differential output voltages (VOUTDIFF) desired. For

example, this could be the full-scale signal that is optimal

for the subsequent ADC’s SNR.

This combination of settings and output range implies

a specific differential input signal (VINDIFF) range, since

VINDIFF = VOUTDIFF/G.

While the input signal’s VINDIFF is fixed when specific

VOUTDIFF and G are chosen, the input signal’s common

mode voltage VICM is not, because the same VINDIFF can

be superimposed on many different VICM values.

The valid VICM range can be set by the swing limits on

+IN and/or –IN, since VICM is the average of +IN and –IN.

It can also be set by internal node swing limits, since

the internal nodes are also operating with common mode

voltage VICM, and these nodes must also be able to swing

enough away from VICM to produce the gained-up output.

On a diamond plot, this valid region of operation for VICM

for a specific output VOUTDIFF is indicated by the portion

of the vertical line going straight up from VOUTDIFF that

falls inside the diamond borders, as shown in Figure 3.

If the part’s input common mode voltage is within the

VICM borders of the diamond, there should be no prob-

lems with clipping. If the differential input signal is shifted

Figure 3. The Blue Arrow Indicates the Range of Valid VICM

Values for VOUTDIFF = –12V, Where No Signals are Clipped,

for the VS = ±15V, G = 16 Case

by a VICM value that is outside of the diamond, either +IN

or –IN (or internal nodes) will be clipped, or the output

itself will hit the rails, and thus result in a clipped output.

The following example shows how a diamond plot point

is determined. For the specific case of VOUTDIFF = –12V

as shown in Figure 3, the upper limit of VICM is 8V, and

the lower limit is –8V.

For VICM = 8V, if the gained-up input (aka output) is –12V,

the maximum negative internal node swing is 6V above

VICM. Referenced to ground, this internal node reaches

8V + 6V = 14V, which is roughly the output high limit of

LTC6373 with ±15V supplies. If VICM were any higher than

8V, the internal node would run into the output high limit,

and the output would clip.

For VICM = –8V, with –12V output, the minimum posi-

tive internal node swing is –6V below VICM. Referenced

to ground, this internal node can hit a minimum of –6V

+ (–8V) = –14V, which is roughly the output low limit of

LTC6373 with ±15V supplies. If VICM were any lower than

−8V, this internal node would run into the output low limit,

and the output would clip.

VS = ±15V

G = 16

DIFFERENTIAL OUTPUT VOLTAGE (V)

–30 –24 –18 –12 –6 0

6 12 18 24 30

–15

–12

–9

–6

–3

6373 F03

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