AD7225CQ AD [Analog Devices], AD7225CQ Datasheet - Page 9

• Manufacturer Part Number: AD7225CQ
• Description: LC2MOS Quad 8-Bit DAC with Separate Reference Inputs
• Manufacturer: AD [Analog Devices]

REV. B
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, V
can be expressed as:
V
where D
DAC latch A. (0
DAC Latch Contents
MSB
1 1 1 1
1 0 0 0
1 0 0 0
0 1 1 1
0 0 0 0
0 0 0 0
OUT
A = V
Table IV. Bipolar (Offset Binary) Code Table
Figure 14. AD7225 Bipolar Output Circuit
A
is a fractional representation of the digital word in
BIAS
1 1 1 1
0 0 0 1
0 0 0 0
1 1 1 1
0 0 0 1
0 0 0 0
Figure 15. AGND Bias Circuit
LSB
+ D
D
A
A
(V
IN
255/256).
)
Analog Output
0 V
–V
–V
–V
V
V
REF
REF
REF
REF
REF
128
127
128
128
128
127
128
128
1
1
–V
REF
OUT
A,
–9–
For a given V
duce the effective V
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 V
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 (V
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 V
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.
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.
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 h
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 16. Applying an AC Signal to the AD7225
DD
– 4 V) and lower (2 V) limits of reference voltage when
Y
IN
, increasing AGND above system GND will re-
Figure 17. Transversal Filter
DD
H
–V
REF
X
which must be at least 4 V to en-
or y
n
k
k 1
N
are actually the non-
h
kXn –k 1
AD7225
DD
REF
and V
A. For
SS