Datasheet

S0165-04

VCM

REFM

REFP

INTREF

EXTREF

4kW

1kW

Internal

Reference

ADS6xxx

Full−scale differential input pp + (Voltage forced on VCM) 1.33

ADS6445, ADS6444

ADS6443, ADS6442

www.ti.com

SLAS531B –MAY 2007–REVISED DECEMBER 2009

Figure 88. Reference Section

Internal Reference

When the device is in internal reference mode, the REFP and REFM voltages are generated internally.

Common-mode voltage (1.5 V nominal) is output on VCM pin, which can be used to externally bias the analog

input pins.

External Reference

When the device is in external reference mode, the VCM acts as a reference input pin. The voltage forced on the

VCM pin is buffered and gained by 1.33 internally, generating the REFP and REFM voltages. The differential

input voltage corresponding to full-scale is given by Equation 2.

(2)

In this mode, the range of voltage applied on VCM should be 1.45 V to 1.55 V. The 1.5-V common-mode voltage

to bias the input pins has to be generated externally.

COARSE GAIN AND PROGRAMMABLE FINE GAIN

ADS644X includes gain settings that can be used to get improved SFDR performance (compared to 0 dB gain

mode). The gain settings are 3.5 dB coarse gain and programmable fine gain from 0 dB to 6 dB. For each gain

setting, the analog input full-scale range scales proportionally, as listed in Table 21.

The coarse gain is a fixed setting of 3.5 dB and is designed to improve SFDR with little degradation in SNR (as

seen in Figure 10 and Figure 11). The fine gain is programmable in 1 dB steps from 0 to 6 dB. With fine gain

also, SFDR improvement is achieved, but at the expense of SNR (there is about 1dB SNR degradation for every

1dB of fine gain).

Product Folder Link(s): ADS6445, ADS6444 ADS6443, ADS6442