LTC1598LCG Просмотр технического описания (PDF) - Linear Technology

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LTC1598LCG

4- and 8-Channel, 3V Micropower Sampling 12-Bit Serial I/O A/D Converters

Linear Technology 

LTC1594L/LTC1598L

APPLICATIONS INFORMATION

Input Leakage Current

Input leakage currents can also create errors if the source

resistance gets too large. For instance, the maximum

input leakage specification of 200nA (at 85°C) flowing

through a source resistance of 600Ω will cause a voltage

drop of 120µV or 0.2LSB. This error will be much

reduced at lower temperatures because leakage drops

rapidly (see typical curve Input Channel Leakage Current

vs Temperature).

REFERENCE INPUTS

The reference input of the LTC1594L/LTC1598L is effec-

tively a 50k resistor from the time CS goes low to the end

of the conversion. The reference input becomes a high

impedance node at any other time (see Figure 9). Since

the voltage on the reference input defines the voltage

span of the A/D converter, the reference input should be

driven by a reference with low ROUT (ex. LT1004, LT1019

and LT1021) or a voltage source with low ROUT.

ROUT

REF+

1

LTC1594L

LTC1598L

VREF

GND

4

1594L/98L F09

Figure 9. Reference Input Equivalent Circuit

Reduced Reference Operation

The effective resolution of the LTC1594L/LTC1598L can

be increased by reducing the input span of the convert-

ers. The LTC1594L/LTC1598L exhibit good linearity and

gain over a wide range of reference voltages (see typical

curves Change in Linearity vs Reference Voltage and

Change in Gain 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 converters.

The following factors must be considered when operat-

ing at low VREF values:

1. Offset

2. Noise

3. Conversion speed (CLK frequency)

Offset with Reduced VREF

The offset of the LTC1594L/LTC1598L has a larger effect

on the output code when the ADCs are operated with

reduced reference voltage. The offset (which is typically

a fixed voltage) becomes a larger fraction of an LSB as the

size of the LSB is reduced. The typical curve of Change in

Offset vs Reference Voltage shows how offset in LSBs is

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

example, a VOS of 122µV which is 0.2LSB with a 2.5V

reference becomes 0.5LSB with a 1V reference and

2.5LSBs with a 0.2V reference. If this offset is unaccept-

able, it can be corrected digitally by the receiving system

or by offsetting the “COM” input of the LTC1594L/

LTC1598L.

Noise with Reduced VREF

The total input referred noise of the LTC1594L/LTC1598L

can be reduced to approximately 400µV 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 2.5V reference, the 400µV noise is

only 0.66LSB peak-to-peak. In this case, the LTC1594L/

LTC1598L 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 jitter in the output code. For example,

with a 1.25V reference this same 400µV noise is 1.32LSB

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 1V, the 400µV noise

becomes equal to 1.65LSBs and a stable code may be

difficult to achieve. In this case, averaging multiple

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.

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