AD7225UQ 데이터 시트보기 (PDF) - Analog Devices
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AD7225UQ Datasheet PDF : 12 Pages
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Figure 14. AD7225 Bipolar Output Circuit
Table IV. Bipolar (Offset Binary) Code Table
DAC Latch Contents
MSB
LSB
Analog Output
1111 1111
1000 0001
+V
REF
127
128
+V
REF
1
128
1000 0000
0111 1111
0000 0001
0000 0000
0V
–V
REF
1
128
–V
REF
127
128
–V
REF
128
128
=
–V REF
AGND BIAS
The AD7225 AGND pin can be biased above system GND
(AD7225 DGND) to provide an offset “zero” analog output
voltage level. Figure 15 shows a circuit configuration to achieve
this for channel A of the AD7225. The output voltage, VOUT A,
can be expressed as:
VOUT A = VBIAS + DA (VIN)
where DA is a fractional representation of the digital word in
DAC latch A. (0 ≤ DA ≤ 255/256).
AD7225
For a given VIN, increasing AGND above system GND will re-
duce the effective VDD–VREF which must be at least 4 V to en-
sure specified operation. Note that because the AGND pin is
common to all four DACs, this method biases up the output
voltages of all the DACs in the AD7225. Note that VDD and VSS
of the AD7225 should be referenced to DGND.
AC REFERENCE SIGNAL
In some applications it may be desirable to have ac reference
signals. The AD7225 has multiplying capability within the up-
per (VDD – 4 V) and lower (2 V) limits of reference voltage when
operated with dual supplies. Therefore ac signals need to be ac
coupled and biased up before being applied to the reference in-
puts. Figure 16 shows a sine wave signal applied to VREF A. For
input signal frequencies up to 50 kHz the output distortion typi-
cally remains less than 0.1%. The typical 3 dB bandwidth figure
for small signal inputs is 800 kHz.
Figure 16. Applying an AC Signal to the AD7225
APPLICATIONS
PROGRAMMABLE TRANSVERSAL FILTER
A discrete-time filter may be described by either multiplication
in the frequency domain or convolution in the time domain i.e.
() () () N
Y ω =H ω X ω
or
yn
=
∑
k =1
hkXn –k+1
The convolution sum may be implemented using the special
structure known as the transversal filter (Figure 17). Basically, it
consists of an N-stage delay line with N taps weighted by N co-
efficients, the resulting products being accumulated to form the
output. The tap weights or coefficients hk are actually the non-
zero elements of the impulse response and therefore determine
the filter transfer function. A particular filter frequency response
is realized by setting the coefficients to the appropriate values.
This property leads to the implementation of transversal filters
whose frequency response is programmable.
Figure 15. AGND Bias Circuit
REV. B
Figure 17. Transversal Filter
–9–
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