ADS1258IPHPREP datasheet(19/57 Pages) TI

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SAMPLE

OFF

AVSS + 1.3V

AIN = ReffB || 2ReffA

AVSS + 1.3V

effA = 190kΩ

effB = 78kΩ

CLK = 16MHz)

effA = 190kΩ

ADCINN

ADCINP

A1 = 0.65pF

B = 1.6pF

A2 = 0.65pF

ADCINN

AVSS + 1.3V

ADCINP

Equivalent

Circuit

eff = tSAMPLE/CX

NOTE: ESD input diodes not shown.

ADS1258-EP

www.ti.com

SBAS445C – MARCH 2009 – REVISED DECEMBER 2009

ADC INPUTS

As with the multiplexer and reference inputs, ESD

diodes protect the ADC inputs. To keep these diodes

The

ADS1258

ADC

inputs

(ADCINP,

ADCINN)

from turning on, make sure the voltages on the input

measure the input signal using internal capacitors

pins do not go below AVSS by more than 100 mV,

that are continuously charged and discharged. The

and likewise do not exceed AVDD by more than 100

left side of Figure 38 shows a simplified schematic of

mV.

the ADC input circuitry; the right side of Figure 38

shows the input circuitry with the capacitors and

switches replaced by an equivalent circuit. Figure 37

shows the ON/OFF timings of the switches shown in

Figure 38. S1 switches close during the input

sampling phase. With S1 closed, CA1 charges to

ADCINP, CA2 charges to ADCINN, and CB charges to

(ADCINP – ADCINN). For the discharge phase, S1

opens first and then S2 closes. CA1 and CA2 discharge

to approximately AVSS + 1.3 V and CB discharges to

0V. This two-phase sample/discharge cycle repeats

with a period of tSAMPLE = 2/fCLK.

Figure 37. S1 and S2 Switch Timing for Figure 38

The charging of the input capacitors draws a transient

current from the source driving the ADS1258 ADC

inputs. The average value of this current can be used

to calculate an effective impedance (Reff) where Reff =

VIN/IAVERAGE. These impedances scale inversely with

fCLK. For example, if fCLK is reduced by a factor of

two, the impedances double.

Figure 38. Simplified ADC Input Structure

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