OP193/OP293

Rev. C | Page 15 of 20

A MICROPOWER FALSE-GROUND GENERATOR

Some single-supply circuits work best when inputs are biased

above ground, typically at ½ of the supply voltage. In these

cases, a false ground can be created by using a voltage divider

buffered by an amplifier. One such circuit is shown in Figure 30.

This circuit generates a false-ground reference at ½ of the supply

voltage, while drawing only about 27 μA from a 5 V supply.

The circuit includes compensation to allow for a 1 μF bypass

capacitor at the false-ground output. The benefit of a large

capacitor is that not only does the false ground present a very

low dc resistance to the load, but its ac impedance is low as well.

The OP193 can both sink and source more than 5 mA, which

improves recovery time from transients in the load current.

OP193

2

3

6

7

5V OR 12V

2.5V OR 6V

+

+

4

10k

Ω

100

Ω

0.022µF

240k

Ω

240k

Ω

1µF

1µF

Figure 30. A Micropower False-Ground Generator

A BATTERY-POWERED VOLTAGE REFERENCE

The circuit of Figure 31 is a battery-powered voltage reference

that draws only 17 μA of supply current. At this level, two AA

alkaline cells can power this reference for more than 18 months.

At an output voltage of 1.23 V at 25°C, drift of the reference is

only 5.5 μV/°C over the industrial temperature range. Load

regulation is 85 μV/mA with line regulation at 120 μV/V.

Design of the reference is based on the Brokaw band gap core

technique. Scaling of Resistor R1 and Resistor R2 produces

creates a temperature-proportional voltage (PTAT), which, in

turn, produces a larger temperature-proportional voltage across

R4 and R5, V1. The temperature coefficient of V1 cancels (first

order) the complementary to absolute temperature (CTAT)

voltage temperature coefficient is at a minimum. Band gap

references can have start-up problems. With no current in R1

and R2, the OP193 is beyond its positive input range limit and

has an undefined output state. Shorting Pin 5 (an offset adjust

pin) to ground forces the output high under these circumstances

and ensures reliable startup without significantly degrading the

OP193’s offset drift.

3

OP193

2

4

5

6

7

Q1

V+

(2.5V TO 36V)

Q2

1

2

3

7

6

5

MAT01AH

R3

68k

Ω

V1

R2

1.5M

Ω

(1.23V @ 25°C)

C1

1000pF

R1

240k

Ω

+

–

+

–

+

–

R4

130k

Ω

R5, 20k

Ω

OUTPUT

ADJUST

Figure 31. A Battery-Powered Voltage Reference

A SINGLE-SUPPLY CURRENT MONITOR

Current monitoring essentially consists of amplifying the voltage

drop across a resistor placed in series with the current to be

measured. The difficulty is that only small voltage drops can be

tolerated, and with low precision op amps, this greatly limits the

overall resolution. The single-supply current monitor of Figure 32

has a resolution of 10 μA and is capable of monitoring 30 mA

of current. This range can be adjusted by changing the current

sense resistor, R1. When measuring total system current, it may

be necessary to include the supply current of the current monitor,

which bypasses the current sense resistor, in the final result.

This current can be measured and calibrated (together with the

residual offset) by adjustment of the offset trim potentiometer,

R2. This produces a deliberate temperature dependent offset.

However, the supply current of the OP193 is also proportional

to temperature, and the two effects tend to track. Voltage devel-

oped at the noninverting input and amplified by (1 + R4/R5)

TO CIRCUIT

UNDER TEST

V+

OP193

3

2

1

5

6

4

7

+

–

R4

9.9k

Ω

R2

100k

Ω

R1

1

Ω

R5

100

Ω

R3

100k

Ω

Figure 32. Single-Supply Current Monitor