ADS1226 查看數據表（PDF） - Analog Devices

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ADS1226

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

Analog Devices 

www.ti.com

Analog Input Measurement Without the Input

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

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

constant charging of the input capacitors presents a

load on the inputs that can be represented by

effective impedances. Figure 22 shows the input

circuitry with the capacitors and switches of

Figure 20 by their effective impedances.

AINPx

AINNx

ESD Protection

AVDD

AVDD/2

AINP S1

CA1

3pF

S2

MUX

AINN S1

CB

6pF

AVDD

S2

CA2

3pF

AVDD/2

Figure 20. Simplified Input Structure with the

Buffer Turned Off

ON

S1

OFF

ON

S2

OFF

tSAMPLE = 12ms

Figure 21. S1 and S2 Switch Timing for Figure 20

ADS1225

ADS1226

SBAS346 – MAY 2006

AINPx

AINNx

AVDD/2

ZeffA = tSAMPLE/CA1 = 4MW

ZeffB = tSAMPLE/CB = 2MW

ZeffA = tSAMPLE/CA2 = 4MW

AVDD/2

Figure 22. Effective Analog Input Impedances

with the Buffer Off

ESD silicon diodes protect the inputs. To keep these

diodes from turning on, make sure the voltages on

the input pins do not go below GND by more than

100mV, and likewise do not exceed VDD by 100mV.

This limitation is shown in Equation 1:

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

(1)

Analog Input Measurement with the Input Buffer

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

shown in Figure 23.

AINP

AINN

1GW

Figure 23. Effective Analog Input Impedances

with the Buffer On

Note 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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