ADS6245 Datasheet, PDF (46/76 Pages) Texas Instruments – DUAL CHANNEL, 14-BIT, 125/105/80/65 MSPS ADC WITH SERIAL LVDS OUTPUTS

ADS6245, ADS6244
ADS6243, ADS6242
SLAS542A – MAY 2007 – REVISED JULY 2007
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VCM
INTREF
Internal
Reference
1 kW
4 kW
EXTREF
INTREF
REFM
REFP
ADS6xxx
S0165-04
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.
Full−scale differential input pp + (Voltage forced on VCM) 1.33
(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
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).
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