www.ti.com

+2.5 V

(+10 V)

U4

OPA277

10 kW

10 kW

5 kW

-

£

2.5 V £

+2.5 V

V

OUT

( 10 V

-

£

£ +10 V)

V

OUT

U2

GND

I

1

OUT

R

FB

V

REF

V

DD

V

DD

V

OUT

I

2

OUT

C

1

OPA277

DAC7821

C

2

Programmable Current Source Circuit

I

L

+

(R2)R3) R1

R3

V

REF

D

(3)

Z

R1 R3

(R1 R2)

R1

(R2

R3

) ) R1 (R2 R3)

(4)

R2¢

15 kW

I

L

LOAD

10 pF

U3

OPA277

R1¢

150 kW

R3¢

50 W

R3

50 W

R1

150 kW

R2

15 kW

OPA277

DAC7821

GND

I

1

OUT

R

FB

V

REF

V

DD

V

DD

V

OUT

I

2

OUT

U1

U2

DAC7821

SBAS365A – OCTOBER 2005 – REVISED FEBRUARY 2006

APPLICATION INFORMATION (continued)

External resistance mismatching is the significant error in Figure 29.

Figure 29. Bipolar Output Circuit

A DAC7821 can be integrated into the circuit in Figure 30 to implement an improved Howland current pump for

precise voltage-to-current conversions. Bidirectional current flow and high voltage compliance are two features of

the circuit. With a matched resistor network, the load current of the circuit is shown by Equation 3:

The value of R3 in the previous equation can be reduced to increase the output current drive of U3. U3 can drive

±20 mA in both directions with voltage compliance limited up to 15 V by the U3 voltage supply. Elimination of the

value of several pF is suggested.

Figure 30. Programmable Bidirectional Current Source Circuit

13