Datasheet

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S0165-04

VCM

REFM

REFP

INTREF

EXTREF

4kW

1kW

Internal

Reference

ADS6xxx

Internal Reference

External Reference

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

(2)

COARSE GAIN AND PROGRAMMABLE FINE GAIN

ADS6245 , ADS6244

ADS6243 , ADS6242

SLAS542A – MAY 2007 – REVISED JULY 2007

Figure 88. Reference Section

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.

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 .

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.

ADS624X 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 shown in Table 22 .

The coarse gain is a fixed setting of 3.5 dB and is designed to improve SFDR with little degradation in SNR. 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): ADS6245 ADS6244 ADS6243 ADS6242