AD7804 datasheet(25/28 Pages) AD

25 / 28 page

AD7804/AD7805/AD7808/AD7809

–25–

REV. A

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.

DVDD

0.01 F

0.1 F

10 F

AD589

+5V

0.1 F

AD780

VIN

GND

6.8k

SERIAL

INTERFACE

AD7804

AVDD

VOUTA

VOUTB

VOUTC

VOUTD

AGND

DGND

LDAC

CLR

COMP

FSIN

SDIN

CLKIN

DVDD

REFOUT

REFIN

Figure 45. Two Externally Applied References

+5V

0.01 F

AD7805

AVDD

DVDD

VOUTA

VOUTB

VOUTC

VOUTD

AGND

DGND

LDAC

CLR

COMP

MODE

DVDD

0.1 F

10 F

0.01 F

IN1

VC1

IN2

VC2

IN3

VC3

IN4

VC4

SSM2164

+15V

–15V

–V

VIN3VIN4

500

560pF

30k

500

560pF

VIN2 VIN1

100pF

30k

+15V

–15V

VOUTA

1/2

OP275

500

560pF

500

560pF

30k

100pF

30k

+15V

–15V

VOUTB

1/2

OP275

Figure 44. Low Cost, Two-Channel Mixer

Low Cost, Two-Channel Mixer Using AD7805, SSM2164 and

OP275

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 k

Ω 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 k

Ω 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.

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