LTC1420IGN datasheet(10/20 Pages) LINER

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LTC1420

Table 2. Comparison of Analog Input Configurations

SUPPLIES

COUPLING

VREF

GAIN

AIN

–

COMMENTS

±5V

4.096V

×±2.048

Best SNR, THD

4.096V

2.5

± 1.024

2.5

Best SINAD, THD for Single Supply

2.048V

2.5

± 1.024

2.5

Worse Noise than Above Case

4.096V

2.5

± 2.048

2.5

Best Single Supply Noise, THD Is Not Optimal

4.096V

0 to 4.096

2.048

Same As Above

±5V

4.096V

×±1.024

±1.024

Very Best SNR, THD

(Transformer)

4.096V

2.5

± 1.024

2.5

± 1.024

Very Best SNR, THD for Single Supply

(Transformer)

1420 F05

µF

+AIN

VSS

VIN

2.5V

LTC1420

–AIN

VCM

1420 F06

+AIN

VSS

VIN

4.096V

2.048V

LTC1420

–AIN

SENSE

Figure 4. DC Coupling a Ground

Centered Signal (Dual Supply System)

Figure 5. DC Coupling a Signal Centered

Around 2.5V (Single Supply System)

Figure 6. DC Coupling a 0V to 4.096V Signal

1420 F07

+AIN

VSS

VIN

µF

LTC1420

–AIN

VCM

Figure 7. AC Coupling to the LTC1420. Note That the Input Signal

Can Almost Always Be Directly Coupled with Better Performance

1420 F04

µF

+AIN

VSS

VIN

LTC1420

–5V

–AIN

VCM

APPLICATIONS INFORMATION

DC Coupling the Input

In most applications the analog input signal can be directly

coupled to the LTC1420 inputs. If the input signal is

centered around ground, such as when dual supply op

amps are used, simply connect – AIN to ground and

connect VSS to –5V (Figure 4). In a single power supply

system with the input signal centered around 2.5V, con-

nect – AIN to VCM and VSS to ground (Figure 5). If the input

signal is not centered around ground or 2.5V, the voltage

for – AIN must be generated externally by a resistor divider

or a voltage reference (Figure 6).

AC Coupling the Input

The analog inputs to the LTC1420 can also be AC coupled

through a capacitor, though in most cases it is simpler to

directly couple the input to the ADC. Figure 7 shows an

example where the input signal is centered around ground

and the ADC operates from a single 5V supply. Note that

the performance would improve if the ADC was operated

from a dual supply and the input was directly coupled (as

in Figure 4). With AC coupling the DC resistance to ground

should be roughly matched for + AIN and – AIN to maintain

offset accuracy.

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