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LTC6912

6912fa

APPLICATIO S I FOR ATIO

Offset Voltage vs Gain Setting

The electrical tables list DC offset (error), VOS(OA), at the

inputs of the internal op amp (See Figure 1). The electrical

tables also show the resulting, gain dependent offset

voltage referred to the INA, or INB pins, VOS(IN). The two

measures are related through the feedback/input resistor

ratio, which equals the nominal gain-magnitude setting,

|GAIN|:

VOS(IN) = (1 + 1/|GAIN|) VOS(OA)

Offset voltages at any gain setting can be inferred from this

relationship. For example, an internal amplifier offset

VOS(OA) of 1mV will appear referred to the INA, INB pins as

2mV at a gain setting of 1, or 1.5mV at a gain setting of 2.

At high gains, VOS(IN) approaches VOS(OA). (Offset voltage

is random and can have either polarity centered on 0V).

The MOS input circuitry of the internal op amp in Figure 1

draws negligible input currents (less than 10

µA), so only

VOS(OA) and the GAIN affect the overall amplifier’s offset.

AC-Coupled Operation

Adding capacitors in series with the INA and INB pins

converts the LTC6912-X into a dual AC-coupled inverting

amplifier, suppressing the input signal’s DC level (and also

adding the additional benefit of reducing the offset voltage

from the LTC6912-X’s amplifier itself). No further compo-

nents are required because the input of the LTC6912-X

biases itself correctly when a series capacitor is added.

The INA and INB analog input pins connect internally to a

resistor whose nominal value varies between 10k

Ω and

Ω depending on the version of LTC6912 used (see the

rightmost column of Tables 1 and 2). Therefore, the low

frequency cutoff will vary with capacitor and gain setting.

If, for example, a low frequency corner of 1kHz (or lower)

on the LTC6912-1 is desired, use a series capacitor of

0.16

µF or larger. 0.16µF has a reactance of 1kΩ at 1kHz,

giving a 1kHz lower –3dB frequency for gain settings of

10V/V through 100V/V. If the LTC6912-1 is operated at

lower gain settings with a 0.16

µF capacitor, the higher

input resistance will reduce the lower corner frequency

down to 100Hz at a gain setting of 1V/V. These frequencies

scale inversely with the value of input capacitor used.

Note that operating the LTC6912 family in “zero” gain

mode (digital state 0000) open circuits both the INA and

INB pins and this demands some care if employed with a

series AC coupling input capacitor. When the chip enters

the zero gain mode, the opened INA or INB pin tends to

sample and freeze the voltage across the capacitor to the

value it held just before the zero gain state. This can place

the INA or INB pin at or near the DC potential of a supply

rail. (The INA or INB pin may also drift to a supply potential

in this state due to small leakage currents.) To prevent

driving the INA or INB pin outside the supply limit and

potentially damaging the chip, avoid AC input signals in

the zero gain state with an AC coupling capacitor. Also,

switching later to a non-zero gain value will cause a

transient pulse at the output of the LTC6912-1 (with a time

constant set by the capacitor value and the new LTC6912-1

input resistance value). This occurs because the INA and

INB pins return to the AGND potential forcing transient

current sourced by the amplifier output to charge the AC

coupling capacitor to its proper DC blocking value.

SNR and Dynamic Range

The term “dynamic range” is much used (and abused)

with signal paths. Signal-to-noise (SNR) is an unambigu-

ous comparison of signal and noise levels, measured in

the same way and under the same operating conditions. In

a variable gain amplifier, however, further characterization

is useful because both noise and maximum signal level in

the amplifier will vary with the gain setting, in general. In

the LTC6912-X, maximum output signal is independent of

gain (and is near the full power supply voltage, as detailed

in the swing sections of the Electrical Characteristics

table). The maximum input level falls with increasing gain,

and the input-referred noise falls as well (listed also in the

table). To summarize the useful signal range in such an

amplifier, we define dynamic range (DR) as the ratio of

maximum input (at unity gain) to minimum input-referred

noise (at maximum gain). This DR has a physical interpre-

tation as the range of signal levels that will experience an

SNR above unity V/V or 0dB. At a 10V total power supply,

DR in the LTC6912-X (gains 0V/V to 100V/V), the DR is

typically 115dB (the ratio of 9.9 VP-P, or 3.5VRMS, maxi-

mum input to the 6.3

µVRMS high gain input noise). The

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