LMH6704_14 Datasheet, PDF (10/20 Pages) National Semiconductor (TI) – MHz Selectable Gain Buffer with Disable

LMH6704
SNOSAD0C – FEBRUARY 2005 – REVISED MARCH 2013
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E_O = _{(4kTRSOURCE + (IBN * RSOURCE)}²_{) * GN2 + (OUTPUT REFERRED NOISE VOLTAGE)}²_{, Where}
G_N = Noise Gain and 4kT = 16E-21 Joules @ Room Temperature
(1)
For example, if an A_V = +2 configuration is used with a source impedance of 37.5Ω (parallel combination of 75Ω
source and 75Ω termination impedances), where “I_BN” is 18.5pA/√Hz and the output referred voltage noise
(excluding non-inverting input noise current) can be found in Table 2. The total noise (E_O) at the output can be
calculated as:
E_O = (16E-21*37.5 + (18.5 pA*37.5)²)*2² + (10.5 nV)² = 10.6 nV/ Hz
(2)
Table 2. Measured Output Noise Voltage⁽¹⁾
Gain (A_V)
+2
+1
+1, alternate method shown in Figure 20
-1
Output Referred Voltage Noise
(nV/√Hz), excluding non-inverting noise current
10.5
9.3
10.5
10.5
(1) Note: f ≥ 100 kHz
ENABLE/DISABLE
SUPPLY
MID-POINT
20 k:
BIAS CIRCUITRY
PIN 6
V_S+
20 k: ^PULL-UP
RESISTOR
V_S+ - V_S-
2
Q₂ Q₁
20 k:
PIN 5
DIS
I TAIL
PIN 2
V_S-
NOTE: PINS 2, 5, 6 ARE EXTERNAL
Figure 22. DIS Pin Simplified Schematic
The LMH6704 has a TTL logic compatible disable function. Apply a logic low (<.8V) to the DS pin and the
LMH6704 is disabled. Apply a logic high (>2.0V), or let the pin float and the LMH6704 is enabled. Voltage, not
current, at the Disable pin (DS) determines the enable/disable state. Care must be exercised to prevent the
disable pin voltage from going more than .8V below the midpoint of the supply voltages (0V with split supplies,
V⁺/2 with single supply biasing). Doing so could cause transistor Q1 to Zener resulting in damage to the disable
circuit (See Figure 22 or the simplified internal schematic diagram using SOT-23 package pin numbers). The
core amplifier is unaffected by this, but the disable operation could become permanently slower as a result.
Disabled, the LMH6704 inputs and output become high impedances. While disabled the LMH6704 quiescent
current is approximately 250 µA. Because of the pull up resistor on the disable circuit, the I_CC and I_EE currents
(positive and negative supply currents respectively) are not balanced in the disabled state. The positive supply
current (I_CC) is approximately 350 µA while the negative supply current (I_EE) is only 250 µA. The remaining I_EE
current of 100 µA flows through the disable pin.
The disable function can be used to create analog switches or multiplexers. Implement a single analog switch
with one LMH6704 positioned between an input and output. Create an analog multiplexer with several
LMH6704’s. Use the circuit shown in for multiplexer applications because there is no RG to shunt signals to
ground.
10
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