THS1401_07 Datasheet, PDF (19/26 Pages) Texas Instruments – 14-Bit, 1/3/8 MSPS, DSP-COMPATIBLE ANALOG-TO-DIGITAL CONVERTERS WITH INTERNAL REFERENCE AND PGA

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APPLICATION INFORMATION
internal/external reference operation
The THS1401/3/8 ADC can either be operated
using the built-in band gap reference or using an
external precision reference in case very high dc
accuracy is needed.
The REF+ and REF+ outputs are given by:
ǒ Ǔ REF )+
VBG
1
)
2
3
and REF–
If the built-in reference is used, VBG equals 1.5 V
which results in REF+ = 2.5 V, REF− = 0.5 V and
∆REF = 2 V.
The internal reference can be disabled by writing
1 to D12 (REF) in the control register (address 3).
The band gap reference is then disconnected and
can be substituted by a voltage on the VBG pin.
programmable gain amplifier
The on-chip programmable gain amplifier (PGA)
has eight gain settings. The gain can be changed
by writing to the PGA gain register (address 1).
The range is 0 to 7dB in steps of one dB.
out of range indication
The OV output of the ADC indicates an out of
range condition. Every time the difference on the
analog inputs exceeds the differential reference,
this signal is asserted. This signal is updated the
same way as the digital data outputs and therefore
subject to the same pipeline delay.
offset compensation
With the offset register it is possible to
automatically compensate system offset errors,
including errors caused by additional signal
THS1401
THS1403
THS1408
SLAS248D − DECEMBER 1999 − REVISED SEPTEMBER 2005
conditioning circuitry. If the offset compensation is
enabled (D7 (OFF) in the control register), the
value in the offset register (address 2) is
automatically added to the output of the ADC.
In order to set the correct value of the offset
compensation register, the ADC result when the
input signal is 0 must be read by the host
processor and written to the offset register
(address 2).
test modes
The ADC core operation can be tested by
selecting one of the available test modes (see
control register description). The test modes
apply various voltages to the differential input
depending on the setting in the control register.
digital I/O
The digital inputs and outputs of the THS1401/3/8
ADC are 3-V CMOS compatible. In order to avoid
current feed back errors, the capacitive load on
the digital outputs should be as low as possible (50
pF max). Series resistors (100 Ω) on the digital
outputs can improve the performance by limiting
the current during output transitions.
The parallel interface of the THS1401/3/8 ADC
features 3-state buffers, making it possible to
directly connect it to a data bus. The output buffers
are enabled by driving the OE input low.
Refer to the read and write timing diagrams in the
parameter measurement information section for
information on read and write access.
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