AD7804/AD7805/AD7808/AD7809
Low Cost, Two-Channel Mixer Using AD7805, SSM2164 and
OP275
Dual External Reference Input Capability
It is possible to operate the AD7804/AD7805/AD7808/AD7809
with two externally applied references. Figure 45 shows the
connections for the AD7804. Reference one, the AD589, is
connected to the REFIN pin of the part; the second reference,
the AD780, is used to overdrive the internal VDD/2 reference
which is provided at the COMP pin of the device. With the
circuit shown in Figure 45 it is possible to configure two of the
channels for operation with the AD780 2.5 V reference and the
other two with the AD589 1.23 V reference. The channel con-
trol register allows the user to select the reference for the indi-
vidual channels.
The SSM2164 is a quad voltage controlled amplifier (VCA)
with 120 dB of gain control range. Each VCA in the package is
a current in, current out device with a –33 mV/dB voltage con-
trol input port. Figure 44 shows a basic application circuit
which can be used to implement a low cost stereo, two channel
mixer. A 30 kΩ resistor converts the input voltage to an input
current for the VCA. The 500 Ω resistor and 560 pF capacitor
on the input are added to ensure stable operation of the
SSM2164. The IOUT pin of the SSM2164 should be maintained
at virtual ground and thus the OP275 is operated in its inverting
mode. Its wide bandwidth, high slew rate and low power make it
ideal for a current to voltage converter. A 30 kΩ feedback resis-
tor is chosen to match the input resistor and thus give unity gain
for a zero volt control voltage input. The 100 pF capacitors
reduce high frequency noise and can be increased to reduce the
low pass cutoff frequency for further noise reduction. The
AD7805 in the circuit is used to control the attenuation of the
VCA, this application circuit only gives attenuation. The voltage
output from the AD7805 provides a low impedance drive to the
SSM2164 so attenuation can be controlled accurately. With a
5 V VDD and a VBIAS of VDD/2 the AD7805 has an LSB size of
approximately 4.5 mV. Therefore, the attenuation can be con-
trolled with a resolution of 0.136 dB/bit and thus 750 codes are
required to provide the full 100 dB of attenuation.
+5V
0.1F
10F
0.1F
6.8k⍀
V
AV
DV
DD
IN
DD
V
O
COMP
REFIN
AD780
GND
0.01F
AD589
AD7804
REFOUT
FSIN
SERIAL
SDIN
V
V
V
V
A
INTERFACE
OUT
OUT
OUT
OUT
CLKIN
B
C
D
DV
DD
CLR
LDAC
DGND AGND
Figure 45. Two Externally Applied References
+5V
V
1
V
2
IN
IN
10F
0.1F
30k⍀
30k⍀
100pF
+15V
500⍀
500⍀
30k⍀
0.01F
0.01F
AV
DV
DD
DD
560pF
560pF
+15V
+V
COMP
1/2
OP275
IN
1
AD7805
V 1
C
V
A
OUT
V
A
B
OUT
IN
2
D9
D0
–15V
100pF
V 2
C
V
OUT
IN
3
MODE
CS
V 3
C
30k⍀
+15V
V
C
OUT
WR
IN
4
1/2
OP275
DV
DD
CLR
V 4
C
V
D
OUT
LDAC
SSM2164
V
B
OUT
DGND AGND
–V
500⍀
500⍀
–15V
560pF
560pF
–15V
30k⍀
30k⍀
V
3 V 4
IN
IN
Figure 44. Low Cost, Two-Channel Mixer
REV. A
–25–
ETC [ ETC ]
