LTC1098ACN8 datasheet(24/32 Pages) LINER

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LTC1096/LTC1096L

LTC1098/LTC1098L

10968fc

APPLICATIONS INFORMATION

related to reference voltage for a typical value of VOS. For

example, a VOS of 2mV which is 0.1LSB with a 5V reference

becomes 0.5LSB with a 1V reference and 2.5LSBs with

a 0.2V reference. If this offset is unacceptable, it can be

corrected digitally by the receiving system or by offsetting

the “–” input of the LTC1096.

Noise with Reduced VREF

The total input referred noise of the LTC1096 can be reduced

to approximately 1mV peak-to-peak using a ground plane,

good bypassing, good layout techniques and minimizing

noise on the reference inputs. This noise is insigniﬁcant

with a 5V reference but will become a larger fraction of

an LSB as the size of the LSB is reduced.

For operation with a 5V reference, the 1mV noise is only

0.05LSB peak-to-peak. In this case, the LTC1096 noise

will contribute virtually no uncertainty to the output code.

However, for reduced references, the noise may become

a signiﬁcant fraction of an LSB and cause undesirable jit-

ter in the output code. For example, with a 1V reference,

this same 1mV noise is 0.25LSB peak-to-peak. This will

reduce the range of input voltages over which a stable

output code can be achieved by 1LSB. If the reference is

further reduced to 200mV, the 1mV noise becomes equal

to 1.25LSBs and a stable code may be difﬁcult to achieve.

In this case averaging readings may be necessary.

This noise data was taken in a very clean setup. Any setup-

induced noise (noise or ripple on VCC, VREF or VIN) will

add to the internal noise. The lower the reference voltage

to be used, the more critical it becomes to have a clean,

noise free setup.

Conversion Speed with Reduced VREF

With reduced reference voltages the LSB step size is

reduced and the LTC1096 internal comparator overdrive

is reduced. Therefore, it may be necessary to reduce

the maximum CLK frequency when low values of VREF

are used.

Input Divider

It is OK to use an input divider on the reference input of

the LTC1096 as long as the reference input can be made

to settle within the bit time at which the clock is running.

When using a larger value resistor divider on the reference

input the “–” input should be matched with an equivalent

resistance.

Bypassing Reference Input with Divider

Bypassing the reference input with a divider is also pos-

sible. However, care must be taken to make sure that the

DC voltage on the reference input will not drop too much

below the intended reference voltage.

AC PERFORMANCE

Two commonly used ﬁgures of merit for specifying the

dynamic performance of the ADCs in digital signal pro-

cessing applications are the signal-to-noise ratio (SNR)

and the effective number of bits (ENOBs).

Signal-to-Noise Ratio

The signal-to-noise ratio (SNR) is the ratio between the

RMS amplitude of the fundamental input frequency to

the RMS amplitude of all other frequency components at

the A/D output. This includes distortion as well as noise

products and for this reason it is sometimes referred to as

signal-to-noise + distortion [S/(N + D)]. The output is band

limited to frequencies from DC to one half the sampling

frequency. Figure 11 shows spectral content from DC to

15.625kHz which is 1/2 the 31.25kHz sampling rate.

Figure 11. This Clean FFT of an 11.8kHz Input Shows

Remarkable Performance for an ADC That Draws Only 100μA

When Sampling at the 31.25kHz Rate

FREQUENCY (kHz)

–60

–30

–20

10968 F11

–70

–80

–120

–100

–10

–40

–50

–90

–110

fSAMPLE = 31.25kHz

fIN = 11.8kHz

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