LTC6906

9

6906fa

APPLICATIO S I FOR ATIO

Achieving the highest accuracy requires controlling po-

tential leakage paths. PC board leakage is aggravated by

both dirt and moisture. Effective cleaning is a good first

step to minimizing leakage, and some PC board manufac-

turers offer high impedance or low leakage processing

options.

Another effective method for controlling leakage is to shunt

the leakage current away from the sensitive node through

a low impedance path. The LTC6906 provides a signal on

the GRD pin for this purpose. Figure 10 shows a PC board

layout that uses the GRD pin and a “guard ring” to absorb

leakage currents. The guard ring surrounds the SET pin

must have no solder mask covering it to be effective. The

GRD pin voltage is held within a few millivolts of the SET

pin voltage, so any leakage path between the SET pin and

the guard ring generates no leakage current.

Bypassing the Power Supply

The LTC6906 has on-chip power supply decoupling that

eliminates the need for an external decoupling capacitor in

most cases. Figure 11 shows a simplified equivalent

circuit of the output driver and on-chip decoupling net-

work. When the output driver switches from low to high,

the 800pF capacitor delivers the current needed to charge

the off-chip capacitive load. Within nanoseconds the sys-

tem power supply recharges the 800pF capacitor.

Figure 9. Supply Current vs Frequency

Figure 10. PC Board Layout with Guard Ring

Figure 11. Simplified Equivalent of the Output Driver

and On-Chip Decoupling Circuit

LTC6906

OUT

GND

DIV

LEAKAGE

CURRENT

NO LEAKAGE

CURRENT

GUARD

RING

GRD

SET

6906 F10

1

2

3

6

5

4

NO SOLDER MASK

OVER THE GUARD RING

6

1

OUT

300

Ω

20

Ω

800pF

2

GND

ESD DIODES

6906 F11

DRIVER

LTC6906-1

DECOUPLING

NETWORK

Figure 12 shows a test circuit for evaluating performance

of the LTC6906 with a highly inductive, 330nH power

supply. Figure 13 shows the effectiveness of the

output waveforms remain well formed.

The extremely low supply current of the LTC6906 allows

operation with substantial resistance in the power supply.

Figure 14 shows a test circuit for evaluating performance

of the LTC6906 with a highly resistive, 100

Ω power

supply. Figure 15 shows the effectiveness of the on-chip

waveforms remain well formed. With a 50pF load, a very

small (2.5%) slow tail can be seen on the rising edge. The

output waveform is still well formed even in this case. The

ability of the LTC6906 to operate with a resistive supply

permits supplying power via a CMOS logic gate or

microcontroller pin. Since the LTC6906 has a turn-on time

of less than 200

µsec, this technique can be used to enable

the device only when needed and further reduce power

consumption.

MASTER OSCILLATOR FREQUENCY (kHz)

0

0

10

30

40

50

800

6906 F09

20

400

200

1000

600

1200

60

÷1

÷3

÷10

70

80