Datasheet AD8011 (Analog Devices) - 10

AD8011
(error current times the open-loop inverting input resistance) that This analysis assumes perfect current sources and infinite transistor results (see Figure 7), a more exact low frequency closed-loop VAs. (Q3, Q4 output conductances are assumed zero.) These transfer function can be described as assumptions result in actual versus model open-loop voltage gain and associated input referred error terms being less accurate for G G A = = low gain (G) noninverting operation at the frequencies below the V G R R G R I F F 1 + × + 1 + + open-loop pole of the AD8011. This is primarily a result of the TO TO AO TO input signal (V P) modulating the output conductances of Q3/Q4, resulting in RI less negative than derived here. For inverting for noninverting (G is positive). operation, the actual versus model dc error terms are relatively G much less. A = V 1 – G RF 1 + +
–90
A T
80
O O
70 –100
for inverting (G is negative).
60 –110 PHASE
where G is the ideal gain as previously described. With R
50 –120
I = TO /AO (open-loop inverting input resistance), the second expression
40 ) –130
(positive G) clearly relates to the classical voltage feedback op amp ⍀
GAIN 30 –140
equation with TO omitted due to its relatively much higher value
20 –150
and thus insignificant effect. AO and TO are the open-loop dc
GAIN (dB –160
voltage and transresistance gains of the amplifier, respectively.
10 PHASE (Degrees) AO(s)
These key transfer variables can be described as
0 –170 –10 –180
R1× g | mf × A | 2
–190
A =
–20
O 1 ( – g × 1)
–30 –200
mc R
1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09 FREQUENCY (Hz)
R1× A | 2| and T = Figure 8. Open-Loop Voltage Gain and Phase O 2
AC TRANSFER CHARACTERISTICS
1 – g × R1 The ac small signal transfer derivations below are based on a R mc = I 2 × simplified single-pole model. Though inaccurate at frequencies Therefore g mf approaching the closed-loop BW (CLBW) of the AD8011 at low where gmc is the positive feedback transconductance (not shown) noninverting external gains, they still provide a fair approxima- and 1/gmf is the thermal emitter resistance of devices D1/D2 and tion and an intuitive understanding of its primary ac small signal Q3/Q4. The gmc × R1 product has a design value that results in a characteristics. negative dc open-loop gain of typically –2500 V/V (see Figure 8). For inverting operation and high noninverting gains, these transfer equations provide a good approximation to the actual
+VS
ac performance of the device. To accurately quantify the V
L
O versus VP relationship, AO(s)
S R L S N
and TO(s) need to be derived. This can be seen by the following
T
nonexpanded noninverting gain relationship
O (s) IE V V A O P O (s) RL CL
G V s ( ) /V s ( ) =
Z L
O P
I I LS
G RF + + 1 A [s] T [s] O O
C R P R F N –VS Z I = OPEN LOOP INPUT IMPEDANCE = CI || RL
with R1× g ×| A2 | Figure 7. Z mf I = Open-Loop Input Impedance A (s) = O 1 – g × R1 Though atypical of conventional CF or VF amps, this negative mc open-loop voltage gain results in an input referred error term Sτ1 (VP–VO/G = G/AO + RF/TO) that will typically be negative for G, 1 – g × R1 mc greater than +3/–4. As an example, for G = 10, A O = –2500, and TO = 1.2 MΩ, results in an error of –3 mV using the AV where R1 is the input resistance to A2/A2B, and τ1 (equal to derivation above. CD ⫻ R1 ⫻ A2) is the open-loop dominate time constant, | A2 | R1 and T s ( ) = × O 2 1 τ + s 1 –10– REV. C Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM PRODUCT DESCRIPTION SPECIFICATIONS DUAL SUPPLY SINGLE SUPPLY ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION ORDERING GUIDE Typical Performance Characteristics THEORY OF OPERATION DC GAIN CHARACTERISTICS AC TRANSFER CHARACTERISTICS DRIVING CAPACITIVE LOADS OPTIMIZING FLATNESS INCREASING BW AT HIGH GAINS DRIVING A SINGLE-SUPPLY A/D CONVERTER LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS Revision History