LT6600-2.5
10
660025fe
APPLICATIONS INFORMATION
VMID can be allowed to oat, but it must be bypassed to
an AC ground with a 0.01μF capacitor or some instability
maybe observed. VMID can be driven from a low impedance
source, provided it remains at least 1.5V above V– and at
least 1.5V below V+. An internal resistor divider sets the
voltage of VMID. While the internal 11k resistors are well
matched, their absolute value can vary by ±20%. This
should be taken into consideration when connecting an
external resistor network to alter the voltage of VMID.
VOCM can be shorted to VMID for simplicity. If a different
common mode output voltage is required, connect VOCM
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at VOCM must be less than or equal
to the mid-supply level. For example, voltage (VOCM) ≤
1.65V on a single 3.3V supply. For power supply voltages
higher than 3.3V the voltage at VOCM can be set above
mid-supply, as shown in Table 1. The voltage on VOCM
should not exceed 1V below the voltage on VMID. VOCM
is a high impedance input.
Table 1. Output Common Range for Various Supplies
SUPPLY
VOLTAGE
DIFFERENTIAL OUT
VOLTAGE SWING
OUTPUT COMMON MODE
RANGE FOR LOW DISTORTION
3V
4VP-P
1.4V ≤ VOCM ≤ 1.6V
2VP-P
1V ≤ VOCM ≤ 1.6V
1VP-P
0.75V ≤ VOCM ≤ 1.6V
5V
8VP-P
2.4V ≤ VOCM ≤ 2.6V
4VP-P
1.5V ≤ VOCM ≤ 3.5V
2VP-P
1V ≤ VOCM ≤ 3.75V
1VP-P
0.75V ≤ VOCM ≤ 3.75V
±5V
9VP-P
–2V ≤ VOCM ≤ 2V
4VP-P
–3.5V ≤ VOCM ≤ 3.5V
2VP-P
–3.75V ≤ VOCM ≤ 3.75V
1VP-P
–4.25V ≤ VOCM ≤ 3.75V
NOTE: VOCM is set by the voltage at this RIN. The voltage at VOCM should
not exceed 1V below the voltage at VMID. To achieve some of the output
common mode ranges shown in the table, the voltage at VMID must be set
externally to a value below mid supply.
The LT6600-2.5 was designed to process a variety of input
signals including signals centered around the mid-sup-
ply voltage and signals that swing between ground and
a positive voltage in a single supply system (Figure 1).
The range of allowable input common mode voltage (the
average of VIN+ and VIN– in Figure 1) is determined by
the power supply level and gain setting (see Electrical
Characteristics).
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the
LT6600-2.5 not only provides lowpass ltering but also
level shifts the common mode voltage of the input signal,
DC currents will be generated through the DC path between
input and output terminals. Minimize these currents to
decrease power dissipation and distortion.
Consider the application in Figure 3. VMID sets the output
common mode voltage of the 1st differential amplier
inside the LT6600-2.5 (see the Block Diagram section) at
2.5V. Since the input common mode voltage is near 0V,
there will be approximately a total of 2.5V drop across
the series combination of the internal 1580Ω feedback
resistor and the external 402Ω input resistor. The result-
ing 1.25mA common mode DC current in each input
path,must be absorbed by the sources VIN+ and VIN–.
VOCM sets the common mode output voltage of the 2nd
differential amplier inside the LT6600-2.5, and therefore
sets the common mode output voltage of the lter. Since,
in the example of Figure 3, VOCM differs from VMID by 0.5V,
an additional 625μA (312μA per side) of DC current will
ow in the resistors coupling the 1st differential amplier
output stage to lter output. Thus, a total of 3.125mA is
used to translate the common mode voltages.
A simple modication to Figure 3 will reduce the DC
common mode currents by 36%. If VMID is shorted toVOCM
the common mode output voltage of both op amp stages
will be 2V and the resulting DC current will be 2mA. Of
course, by AC-coupling the inputs of Figure 3, the common
mode DC current can be reduced to 625μA.