LTC6910-1

LTC6910-2/LTC6910-3

21

6910123fa

TYPICAL APPLICATIO

Figure 7 shows a block diagram and Figure 8 the practical

circuit for a low noise amplifier with gain and bandwidth

independently programmable over 100:1 ranges. One

LTC6910-X controls the gain and another controls the

bandwidth. An LT1884 dual op amp forms an integrating

lowpass loop with capacitor C2 to set the programmable

upper corner frequency. The LT1884 also supports rail-to-

rail output swings over the total supply voltage range of

2.7V to 10.5V. AC coupling through capacitor C1 estab-

C1

GAIN CONTROL PGA

(GAIN A)

BANDWIDTH CONTROL PGA

(GAIN B)

GAIN = –1

C2

R1

R2

6910 F05

1

2

πR1C1 ≤

BANDWIDTH

≤

1

R2

(GAIN B)

2

π

C2

Figure 7. Block Diagram of an AC Amplifier with Programmable Gain and Bandwidth

lishes a fixed low frequency corner of 1Hz, which can be

adjusted by changing C1. Alternatively, shorting C1 makes

the amplifier DC coupled. (If DC gain is not needed,

however, the AC coupling suppresses several error sources:

any shifts in DC levels, low frequency noise and all

amplifier DC offset voltages other than the low internally

trimmed LT1884 offset in the integrating amplifier. If

desired, another coupling capacitor in series with the input

can relax the requirements on DC input level as well.)