LTC1197CS8 datasheet(16/28 Pages) LINER

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LTC1197/LTC1197L

LTC1199/LTC1199L

APPLICATI

I FOR ATIO

Mixed Supplies

It is possible to have a microprocessor running off a 5V

supply and communicate with the ADC operating on 3V or

9V supplies. The requirement to achieve this is that the

outputs of CS, CLK and DIN from the MPU have to be able

to trip the equivalent inputs of the ADC and the output of

the ADC must be able to toggle the equivalent input of the

MPU (see typical curve of Digital Input Logic Threshold vs

Supply Voltage). With the LTC1197 operating on a 9V

supply, the output of DOUT may go between 0V and 9V. The

9V output may damage the MPU running off a 5V supply.

The way to solve this problem is to have a resistor divider

on DOUT (Figure 4) and connect the center point to the

MPU input. It should be noted that to get full shutdown, the

CS input of the ADC must be driven to the VCC voltage. This

would require adding a level shift circuit to the CS signal

in Figure 4.

SAMPLE-AND-HOLD

The LTC1197/LTC1197L/LTC1199/LTC1199L provide a

built-in sample-and-hold (S /H) function to acquire sig-

nals. The S /H of the LTC1197/LTC1197L acquires input

signals for the “+” input relative to the “–” input during the

tSMPL time(seeFigure1).Howeverthe S /HoftheLTC1199/

LTC1199L can sample input signals from the “+” input

relative to ground and from the “–” input relative to ground

in addition to acquiring signals from the “+” input relative

to the “–” input (see Figure 5) during tSMPL.

Single-Ended Inputs

The sample-and-hold of the LTC1199/LTC1199L allows

conversion of rapidly varying signals. The input voltage is

sampled during the tSMPL time as shown in Figure 5. The

sampling interval begins as the ODD/SGN bit is shifted in

and continues until the falling CLK edge after the dummy

bit is received. On this falling edge, the S/H goes into hold

mode and the conversion begins.

Differential Inputs

With differential inputs, the ADC no longer converts just a

single voltage but rather the difference between two volt-

ages. In this case, the voltage on the selected “+” input is

still sampled and held and therefore may be rapidly time

varying just as in single-ended mode. However, the volt-

age on the selected “–” input must remain constant and be

free of noise and ripple throughout the conversion time.

Otherwise, the differencing operation may not be per-

formed accurately. The conversion time is 10.5 CLK cycles.

Therefore, a change in the “–” input voltage during this

interval can cause conversion errors. For a sinusoidal

voltage on the “–” input this error would be:

VERROR (MAX) = VPEAK • 2 • π • f(“–”) • 10.5/fCLK

Where f(“–”) is the frequency of the “–” input voltage,

VPEAK is its peak amplitude and fCLK is the frequency of the

CLK. In most cases VERROR will not be significant. For a

60Hz signal on the “–” input to generate a 1/4LSB error

(1.22mV) with the converter running at CLK = 7.2MHz, its

peak value would have to be 2.22V.

+IN

–IN

GND

VCC

CLK

DOUT

VREF

4.7k

4.7

µF

MPU

(e.g. 8051)

P1.4

P1.3

P1.2

1197/99 F04

DIFFERENTIAL INPUTS

COMMON MODE RANGE

0V TO 6V

LTC1197

OPTIONAL

LEVEL SHIFT

Figure 4. Interfacing a 9V-Powered LTC1197 to a 5V System

BOARD LAYOUT CONSIDERATIONS

Grounding and Bypassing

The LTC1197/LTC1197L/LTC1199/LTC1199L should be

used with an analog ground plane and single point ground-

ing techniques. The GND pin should be tied directly to the

ground plane. The VCC pin should be bypassed to the

ground plane using a 1

µF tantalum capacitor with leads as

short as possible. All analog inputs should be referenced

directly to the single point ground. Digital inputs and

outputs should be shielded from and/or routed away from

the reference and analog circuitry.

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