AD7845

REV. B

–8–

APPLI

PROGRAMMABLE GAIN AMPLIFIER (PGA)

The AD7845 performs a PGA function when connected as in

Figure 15. In this configuration, the R-2R ladder is connected

tor. As the code decreases, the R-2R ladder resistance increases

and so the gain increases.

R

DAC

D

×

1

R

FB

,

D

= 0 to

4095

4096









R

DAC

D

×

1

R

DAC

=

–V

IN

D

Figure 15. AD7845 Connected as PGA

As the programmed gain increases, the error and noise also

increase. For this reason, the maximum gain should be limited

to 256. Table III shows gain versus code.

possible to use combinations of the other application resistors,

gain range for the same codes of Table II now goes from l/2

to 128.

Table III. Gain and Error vs. Input Code for Figure 15

Digital Inputs

Gain

Error (%)

1111

1111

1111

4096/4095

≈ 10.04

1000

0000

0000

2

0.07

0100

0000

0000

4

0.13

0010

0000

0000

8

0.26

0001

0000

0000

16

0.51

0000

1000

0000

32

1.02

0000

0100

0000

64

2.0

0000

0010

0000

128

4.0

0000

0001

0000

256

8.0

PROGRAMMABLE CURRENT SOURCES

The AD7845 is ideal for designing programmable current

sources using a minimum of external components. Figures 16

and 17 are examples. The circuit of Figure 16 drives a program-

Figure 17 shows the circuit for sinking a programmable current,

D

×|V

R

DAC

,

D

= 0 to

4095

4096









1

R1

D

×|V

R

DAC









D

×|V

R1

D

×|V

R1

+

D

×|V

R

DAC

=

D

×|V

R1

× 1 +

R1

R

DAC









temperature variations. Now, the only resistor determining load

If R1 = 100

Ω, then the programming range is 0 mA to 100 mA,

and the resolution is 0.024 mA.

Figure 16. Programmable Current Source