Datasheet
165 mA Fs
125 MSPS
S0165-05
VCM
REFM
REFP
INTREF
INTREF
EXTREF
1kW
4kW
Internal
Reference
ADS62P4x
ADS62P45, ADS62P44
ADS62P43, ADS62P42
www.ti.com
SLAS561C –JULY 2007– REVISED FEBRUARY 2012
Input Common-Mode
To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1-μF low-inductance capacitor
connected to ground. The VCM pin is designed to directly drive the ADC inputs. The input stage of the ADC
sinks a common-mode current in the order of 165 μA (at 125 MSPS). Equation 1 describes the dependency of
the common-mode current and the sampling frequency.
(1)
This equation helps to design the output capability and impedance of the VCM driving circuit accordingly.
REFERENCE
ADS62P4X has built-in internal references REFP and REFM, requiring no external components. Design schemes
are used to linearize the converter load seen by the references; this and the on-chip integration of the requisite
reference capacitors eliminates the need for external decoupling. The full-scale input range of the converter can
be controlled in the external reference mode as explained below. The internal or external reference modes can
be selected by programming the serial interface register bit ( REF).
Figure 89. 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 in Equation 2.
Full-scale differential input pp = (Voltage forced on VCM) × 1.33 (2)
In this mode, the 1.5-V common-mode voltage to bias the input pins has to be generated externally.
Copyright © 2007–2012, Texas Instruments Incorporated Submit Documentation Feedback 51
Product Folder Link(s): ADS62P45, ADS62P44 ADS62P43, ADS62P42