ADS1225IRGVT datasheet(11/30 Pages) TI1

Numer części	ADS1225IRGVT

Szczegółowy opis	24-Bit Analog-to-Digital Converter with One- and Two-Channel Differential Inputs and Internal Oscillator

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Producent	TI1 [Texas Instruments]

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Analog Input Measurement Without the Input

Zeff =t

/C

A

SAMPLE

A1 =4MW

Zeff =t

/C

B

SAMPLE

B =2MW

Zeff =t

/C

A

SAMPLE

A2 =4MW

AINPx

AINNx

AVDD/2

GND * 100mV t (AINP, AINN) t VDD ) 100mV

Analog Input Measurement with the Input Buffer

MUX

AVDD

AVDD/2

S

1

S1

AINN

AINP

S

2

C

A1

3pF

C

B

6pF

C

A2

3pF

AVDD

ESDProtection

AINPx

AINNx

S2

AINP

AINN

1GW

ON

OFF

ON

S

1

S

2

OFF

t

=12 s

m

SAMPLE

ADS1225

ADS1226

www.ti.com ....................................................................................................................................................... SBAS346C – MAY 2006 – REVISED JANUARY 2009

Buffer

With the buffer disabled by setting the BUFEN pin

low, the ADS1225 and ADS1226 measure the input

signal using internal capacitors that are continuously

charged

and

discharged.

Figure

20

shows

a

simplified schematic of the ADS1225/6 input circuitry,

with Figure 21 showing the on/off timings of the

switches. The S1 switches close during the input

sampling phase. With S1 closed, CA1 charges to

AINP, CA2 charges to AINN, and CB charges to (AINP

– AINN). For the discharge phase, S1 opens first and

then

S2

closes.

CA1

and

CA2

discharge

to

Figure 22. Effective Analog Input Impedances

approximately VDD/2 and CB discharges to 0V. The

with the Buffer Off

constant charging of the input capacitors presents a

load on the inputs that can be represented by

ESD diodes protect the inputs. To keep these diodes

effective impedances. Figure 22 shows the input

from turning on, make sure the voltages on the input

circuitry with the capacitors and switches of Figure 20

pins do not go below GND by more than 100mV, and

by their effective impedances.

likewise do not exceed VDD by 100mV. This

limitation is shown in Equation 1:

(1)

When the buffer is enabled by setting the BUFEN pin

high, a low-drift, chopper-stabilized input buffer is

used to achieve very high input impedance. The

buffer charges the input sampling capacitors, thus

removing the load from the measurement. Because

the input buffer is chopper-stabilized, the charging of

parasitic capacitances causes the charge to be

carried

away,

as

if

by

resistance.

The

input

impedance can be modeled by a single resistor, as

Figure 20. Simplified Input Structure with the

shown in Figure 23. Note that when START is low,

Buffer Turned Off

the buffer must be disabled to prevent loading of the

inputs.

Figure 23. Effective Analog Input Impedances

with the Buffer On

Figure 21. S1 and S2 Switch Timing for Figure 20

Note also that the analog inputs (listed in the

Electrical Characteristics table as Absolute Input

Range) must remain between GND + 0.05V to AVDD

– 1.5V. Exceeding this range degrades linearity and

results in performance outside the specified limits.

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Product Folder Link(s): ADS1225 ADS1226

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