THS1408-EP_14 Datasheet, PDF (21/28 Pages) Texas Instruments – 14-BIT, 3/8 MSPS DSP-COMPATIBLE ANALOG-TO-DIGITAL CONVERTER WITH INTERNAL REFERENCE AND PGA

THS1408-EP
14-BIT, 3/8 MSPS DSP-COMPATIBLE ANALOG-TO-DIGITAL CONVERTER
WITH INTERNAL REFERENCE AND PGA
SGLS129B − JULY 2002 − REVISED FEBRUARY 2010
APPLICATION INFORMATION
internal/external reference operation
The THS1408 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 ǒ Ǔ _{REF )+}
VBG
1
)
2
3
and REF– + VBG
1–
2
3
If the built-in reference is used, VBG equals 1.5 V, which results in REF+ = 2.5 V, REF− = 0.5 V and ΔREF = 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 7 dB 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 conditioning circuitry. If the offset compensation is enabled (D7 (OFF) in the control register), the
value in the offset register (address 2) is automatically subtracted from 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 THS1408 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 THS1408 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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