THS1040

SLAS290C − OCTOBER 2001 − REVISED OCTOBER 2004

25

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APPLICATION INFORMATION

C1

8 pF

AVDD

AGND

VCM = AIN+/AIN− Common Mode Voltage

AIN

_

+

IIN

R2 = 1/CS fCLK

Figure 33. Equivalent Circuit for the AIN Switched Capacitor Input

AIN input damping

The charging current pulses into AIN+ and AIN− can make the signal sources jump or ring, especially if the

sources are slightly inductive at high frequencies. Inserting a small series resistor of 20

Ω or less and a small

capacitor to ground of 20 pF or less in the input path can damp source ringing (see Figure 34). The resistor and

capacitor values can be made larger than 20

Ω and 20 pF if reduced input bandwidth and a slight gain error (due

to potential division between the external resistors and the AIN equivalent resistors) are acceptable.

Note that the capacitors should be soldered to a clean analog ground with a common ground point to prevent

any voltage drops in the ground plane appearing as a differential voltage at the ADC inputs.

AIN

R < 20

Ω

VS

C < 20 pF

Figure 34. Damping Source Ringing Using a Small Resistor and Capacitor

driving the VREF pin

Figure 35 shows the equivalent load on the VREF pin when driving the ADC internal references buffer via this

AVDD

AGND

(AVDD + VREF) /4

VREF

RIN

10 k

Ω

REFSENSE = AVDD,

MODE = AVDD/2 or AVDD

_

+

MODE = AVDD

Figure 35. Equivalent Circuit of VREF

I

REF +

3V

REF *

AV

DD

4

R

IN

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