Datasheet

AVDD

GND

REF

REFGND

ADS7947

ADS7948

ADS7949

REF50xx

(1)

AVDD

AGND

1 Fm

Ceramic

ADS7947

ADS7948

ADS7949

SLAS708 –SEPTEMBER 2010

www.ti.com

REFERENCE

The ADS7947/8/9 use an external reference voltage during the conversion of a sampled signal. The devices

switch the capacitors used in the conversion process to the reference terminal during conversion. The switching

frequency is the same as the SCLK frequency. It is necessary to decouple the REF terminal to REFGND with a

1µF ceramic capacitor in order to get the best noise performance from the device. The capacitor must be placed

closest to these pins. The reference input can be driven with the REF50xx series precision references from TI.

Figure 41 shows a typical reference driving circuit.

Sometimes it is convenient to use AVDD as a reference. The ADS794x allow reference ranges up to AVDD.

However, make sure that AVDD is well-bypassed and that there is a separate bypass capacitor between REF

and REFGND.

(1) Select the appropriate device as described by the required reference value. For example, select the REF5040 for a 4V reference, the

REF5030 for a 3V reference, and the REF5025 for a 2.5V reference. Ensure that (AVDD – REF) > 0.2V so that the REF50xx functions

properly.

Figure 41. Typical Reference Driving Circuit

CLOCK

The ADS794x use SCLK for conversions (typically 34MHz). A lower frequency SCLK can be used for

applications requiring sample rates less than 2MSPS. However, it is better to use a 34MHz SCLK and slow down

the device speed by choosing a lower frequency for CS, which allows more acquisition time. This configuration

relaxes constraints on the output impedance of the driving circuit. Refer to the Application Information section for

calculation of the driving circuit output impedance.

Product Folder Link(s): ADS7947 ADS7948 ADS7949