LTC1196 datasheet(20/28 Pages) LINER

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LTC1196/LTC1198

APPLICATI

I FOR ATIO

“–” Input Settling

At the end of the tSMPL, the input capacitor switches to the

“–” input and conversion starts (see Figures 1 and 7).

During the conversion, the “+” input voltage is effectively

“held” by the sample-and-hold and will not affect the

conversion result. However, it is critical that the “–” input

voltage settle completely during the first CLK cycle of the

conversion time and be free of noise. Minimizing RSOURCE–

will improve settling time. If a large “–” input source

resistance must be used, the time allowed for settling can

be extended by using a slower CLK frequency.

Input Op Amps

When driving the analog inputs with an op amp it is

important that the op amp settle within the allowed time

(see Figures 1 and 7). Again, the “+” and “–” input

sampling times can be extended as described above to

accommodate slower op amps.

To achieve the full sampling rate, the analog input should

be driven with a low impedance source (<100

Ω) or a high

speed op amp (e.g., the LT1223, LT1191, or LT1226).

Higher impedance sources or slower op amps can easily

be accommodated by allowing more time for the analog

input to settle as described above.

Source Resistance

The analog inputs of the LTC1196/LTC1198 look like a

25pF capacitor (CIN) in series with a 120Ω resistor (RON)

as shown in Figure 8. CIN gets switched between the

selected “+” and “–” inputs once during each conversion

cycle. Large external source resistors will slow the settling

of the inputs. It is important that the overall RC time

constants be short enough to allow the analog inputs to

completely settle within tSMPL.

Figure 8. Analog Input Equivalent Circuit

REFERENCE INPUT

The voltage on the reference input of the LTC1196 defines

the voltage span of the A/D converter. The reference input

has transient capacitive switching currents which are due

to the switched-capacitor conversion technique (see Fig-

ure 9). During each bit test of the conversion (every CLK

cycle), a capacitive current spike will be generated on the

reference pin by the ADC. These high frequency current

spikes will settle quickly and do not cause a problem if the

reference input is bypassed with at least a 0.1

µF capacitor.

The reference input can be driven with standard voltage

references. Bypassing the reference with a 0.1

µFcapacitor

is recommended to keep the high frequency impedance

low as described above. Some references require a small

resistor in series with the bypass capacitor for frequency

stability. See the individual reference data sheet for details.

Figure 9. Reference Input Equivalent Circuit

Reduced Reference Operation

The minimum reference voltage of the LTC1198 is limited

to 2.7V because the VCC supply and reference are inter-

nally tied together. However, the LTC1196 can operate

with reference voltages below 1V.

The effective resolution of the LTC1196 can be increased

by reducing the input span of the converter. The LTC1196

exhibits good linearity and gain over a wide range of

reference voltages (see typical curves of Linearity and Full-

Scale Error vs Reference Voltage). However, care must be

taken when operating at low values of VREF because of the

reduced LSB step size and the resulting higher accuracy

requirement placed on the converter. The following factors

must be considered when operating at low VREF values.

1. Offset

2. Noise

RON

120

Ω

CIN

25pF

LTC1196

LTC1198

“+”

INPUT

RSOURCE

VIN +

“–”

INPUT

RSOURCE

–

VIN –

1196/98 F08

↑tSMPL

tSMPL↓

RON

5pF TO

30pF

LTC1196

REF+

ROUT

VREF

EVERY CLK CYCLE

GND

1196/98 F09

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