REV. B

OP470

–13–

10

0%

100

90

Figure 14. Noise Density of Low Noise Amplifier, G = 1000

DIGITAL PANNING CONTROL

Figure 15 uses a DAC-8408, quad 8-bit DAC to pan a signal

between two channels. The complementary DAC current out-

puts two of the DAC-8408’s four DACs drive current-to-voltage

converters built from a single quad OP470. The amplifiers have

complementary outputs with the amplitudes dependent upon

the digital code applied to the DAC. Figure 16 shows the comple-

mentary outputs for a 1 kHz input signal and digital ramp applied

to the DAC data inputs. Distortion of the digital panning con-

trol is less than 0.01%.

10

0%

100

90

1ms

5V

5V

A OUT

A OUT

Figure 16. Digital Panning Control Output

Gain error due to the mismatching between the internal DAC

ladder resistors and the current-to-voltage feedback resistors is

eliminated by using feedback resistors internal to the DAC. Of

the four DACs available in the DAC-8408, only two DACs, A

and C, actually pass a signal. DACs B and D are used to pro-

vide the additional feedback resistors needed in the circuit. If

the VREFB and VREFD inputs remain unconnected, the

current-to-voltage converters using RFBB and RFBD are unaf-

fected by digital data reaching DACs B and D.

1/4

OP470E

+15V

–15V

1/4

OP470E

A OUT

20pF

A OUT

20pF

DAC A

DAC B

1/4

OP470E

B OUT

20pF

DAC C

1/4

OP470E

B OUT

20pF

DAC D

DAC SELECT

1k

1k

5V

DGND

DS2

DS1

R/W

A/B

DAC-8408GP

SIDE A IN

SIDE B IN

DAC DATA BUS

PINS 9 (LSB) – 16 (MSB)

5V

Figure 15. Digital Panning Control Circuit